Aer backends do not have a coupling_map
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### Informations

- **Qiskit Aer version**: master
- **Python version**:
- **Operating system**:

### What is the current behavior?
According to the spec "Qiskit Backend Specifications for OpenQASM and OpenPulse Experiments", all backends are required to have a `coupling_map` in their `backend.configuration()`.  The `Aer` backends do not.


### Steps to reproduce the problem



### What is the expected behavior?



### Suggested solutions



diff --git a/qiskit/providers/aer/backends/qasm_simulator.py b/qiskit/providers/aer/backends/qasm_simulator.py
index e9574417..f6234ae0 100644
--- a/qiskit/providers/aer/backends/qasm_simulator.py
+++ b/qiskit/providers/aer/backends/qasm_simulator.py
@@ -90,7 +90,7 @@ class QasmSimulator(AerBackend):
             cores. For systems with a small number of cores it enabling
             can reduce performance (Default: False).
 
-        * "ch_approximation_error" (double): Set the error in the 
+        * "ch_approximation_error" (double): Set the error in the
             approximation for the ch method. A smaller error needs more
             memory and computational time. (Default: 0.05)
 
@@ -129,7 +129,8 @@ class QasmSimulator(AerBackend):
         'open_pulse': False,
         'memory': True,
         'max_shots': 100000,
-        'description': 'A C++ simulator with realistic for qobj files',
+        'description': 'A C++ simulator with realistic noise for qobj files',
+        'coupling_map': None,
         'basis_gates': ['u1', 'u2', 'u3', 'cx', 'cz', 'id', 'x', 'y', 'z',
                         'h', 's', 'sdg', 't', 'tdg', 'ccx', 'swap',
                         'snapshot', 'unitary'],
@@ -217,13 +218,12 @@ class QasmSimulator(AerBackend):
                     else:
                         if n_qubits > 63:
                             raise AerError(err_string + ', and has too many ' +
-                                           'qubits to fall back to the ' + 
+                                           'qubits to fall back to the ' +
                                            'CH simulator.')
                         if not ch_supported:
                             raise AerError(err_string + ', and contains ' +
-                                           'instructions not supported by ' + 
+                                           'instructions not supported by ' +
                                            'the CH simulator.')
                         logger.info('The QasmSimulator will automatically '
                                     'switch to the CH backend, based on '
                                     'the memory requirements.')
-
diff --git a/qiskit/providers/aer/backends/statevector_simulator.py b/qiskit/providers/aer/backends/statevector_simulator.py
index 2ad7a51f..5ad64aef 100644
--- a/qiskit/providers/aer/backends/statevector_simulator.py
+++ b/qiskit/providers/aer/backends/statevector_simulator.py
@@ -80,6 +80,7 @@ class StatevectorSimulator(AerBackend):
         'memory': True,
         'max_shots': 1,
         'description': 'A C++ statevector simulator for qobj files',
+        'coupling_map': None,
         'basis_gates': ['u1', 'u2', 'u3', 'cx', 'cz', 'id', 'x', 'y', 'z',
                         'h', 's', 'sdg', 't', 'tdg', 'ccx', 'swap',
                         'snapshot', 'unitary'],
diff --git a/qiskit/providers/aer/backends/unitary_simulator.py b/qiskit/providers/aer/backends/unitary_simulator.py
index da7ec99e..cbad7534 100644
--- a/qiskit/providers/aer/backends/unitary_simulator.py
+++ b/qiskit/providers/aer/backends/unitary_simulator.py
@@ -60,12 +60,12 @@ class UnitarySimulator(AerBackend):
             (Default: 6).
     """
 
-    MAX_QUBITS_MEMORY = int(log2(sqrt(local_hardware_info()['memory'] * (1024 ** 3) / 16)))
+    MAX_QUBIT_MEMORY = int(log2(sqrt(local_hardware_info()['memory'] * (1024 ** 3) / 16)))
 
     DEFAULT_CONFIGURATION = {
         'backend_name': 'unitary_simulator',
         'backend_version': __version__,
-        'n_qubits': MAX_QUBITS_MEMORY,
+        'n_qubits': MAX_QUBIT_MEMORY,
         'url': 'https://github.com/Qiskit/qiskit-aer',
         'simulator': True,
         'local': True,
@@ -74,7 +74,8 @@ class UnitarySimulator(AerBackend):
         'memory': False,
         'max_shots': 1,
         'description': 'A Python simulator for computing the unitary' +
-                        'matrix for experiments in qobj files',
+                       'matrix for experiments in qobj files',
+        'coupling_map': None,
         'basis_gates': ['u1', 'u2', 'u3', 'cx', 'cz', 'id', 'x', 'y', 'z',
                         'h', 's', 'sdg', 't', 'tdg', 'ccx', 'swap',
                         'snapshot', 'unitary'],
['qiskit/providers/aer/backends/qasm_simulator.py', 'qiskit/providers/aer/backends/unitary_simulator.py', 'qiskit/providers/aer/backends/statevector_simulator.py']