Houdini: hbatch errors on start within OpenPype environment on import hdefereval
**Describe the bug**

The [OpenPype Houdini pipeline imports `hdefereval`](https://github.com/pypeclub/OpenPype/blob/develop/openpype/hosts/houdini/api/pipeline.py#L7) which can't be imported outside a graphical Houdini.

**Error**
```python
Error running pythonrc.py:
Traceback (most recent call last):
  File "path/to/openpype/hosts/houdini/startup/python3.7libs/pythonrc.py", line 2, in <module>
    from openpype.hosts.houdini import api
  File "path/to/openpype/hosts/houdini/api/__init__.py", line 1, in <module>
    from .pipeline import (
  File "path/to/openpype/hosts/houdini/api/pipeline.py", line 7, in <module>
    import hdefereval
  File "C:/PROGRA~1/SIDEEF~1/HOUDIN~1.455/houdini/python3.7libs/hdefereval.py", line 242, in <module>
    raise ImportError("%s is only available in a graphical Houdini" % __name__)
ImportError: hdefereval is only available in a graphical Houdini
```

**To Reproduce**
Steps to reproduce the behavior:
1. Run e.g. `hbatch` from within the OpenPype Houdini environment.

E.g. when running houdini from OpenPype, from Houdini launch hbatch:
```python
import subprocess
subprocess.Popen(["hbatch"])
```

**Expected behavior**

Launch without errors.
diff --git a/openpype/hosts/houdini/api/pipeline.py b/openpype/hosts/houdini/api/pipeline.py
index d079c9ea81..31c82b1cfd 100644
--- a/openpype/hosts/houdini/api/pipeline.py
+++ b/openpype/hosts/houdini/api/pipeline.py
@@ -4,7 +4,6 @@ import logging
 import contextlib
 
 import hou
-import hdefereval
 
 import pyblish.api
 import avalon.api
@@ -305,7 +304,13 @@ def on_new():
         start = hou.playbar.playbackRange()[0]
         hou.setFrame(start)
 
-    hdefereval.executeDeferred(_enforce_start_frame)
+    if hou.isUIAvailable():
+        import hdefereval
+        hdefereval.executeDeferred(_enforce_start_frame)
+    else:
+        # Run without execute deferred when no UI is available because
+        # without UI `hdefereval` is not available to import
+        _enforce_start_frame()
 
 
 def _set_context_settings():
['openpype/hosts/houdini/api/pipeline.py']