MSOutput/MSUnmerged fails to get an instance of the MongoDB object
**Impact of the bug**
MSOutput and MSUnmerged

**Describe the bug**
With the changes and the new `mongomock` library provided in this PR: https://github.com/dmwm/WMCore/pull/11007

microservices that depend on the MongoDB class will fail to run because the MicroServices spec does not have this new dependency, as can be seen in the reqmgr2ms.spec: https://github.com/cms-sw/cmsdist/blob/comp_gcc630/reqmgr2ms.spec

making the service to fail to start the REST manager and serve RESTful requests, see traceback in [1]

**How to reproduce it**
Deploy reqmgr2ms 2.0.1.pre3 or 2.0.1.pre4 and start reqmgr2ms services.

**Expected behavior**
The `mongomock` library should not be requested to run microservices (be it production service or not). It's only necessary for the unit tests. So, my suggestion is to enclose that import in a try/except clause and simply bypass it when the library is not found.

**Additional context and error message**
[1]
```
ERROR initializing MicroService REST module.
Traceback (most recent call last):
  File "/data/srv/HG2203a-939cb157ff04435ac3077ba2f0a76194/sw.amaltaro/slc7_amd64_gcc630/cms/reqmgr2ms/1.0.1.pre3-082edd71c26e8df9fb2c59ca16cef404/lib/python3.8/site-packages/WMCore/MicroService/Service/Data.py", line 58, in __init__
    self.mgr = getattr(module, cname)(config)
  File "/data/srv/HG2203a-939cb157ff04435ac3077ba2f0a76194/sw.amaltaro/slc7_amd64_gcc630/cms/reqmgr2ms/1.0.1.pre3-082edd71c26e8df9fb2c59ca16cef404/lib/python3.8/site-packages/WMCore/MicroService/MSManager.py", line 107, in __init__
    from WMCore.MicroService.MSOutput.MSOutput import MSOutput
  File "/data/srv/HG2203a-939cb157ff04435ac3077ba2f0a76194/sw.amaltaro/slc7_amd64_gcc630/cms/reqmgr2ms/1.0.1.pre3-082edd71c26e8df9fb2c59ca16cef404/lib/python3.8/site-packages/WMCore/MicroService/MSOutput/MSOutput.py", line 25, in <module>
    from WMCore.Database.MongoDB import MongoDB
  File "/data/srv/HG2203a-939cb157ff04435ac3077ba2f0a76194/sw.amaltaro/slc7_amd64_gcc630/cms/reqmgr2ms/1.0.1.pre3-082edd71c26e8df9fb2c59ca16cef404/lib/python3.8/site-packages/WMCore/Database/MongoDB.py", line 12, in <module>
    import mongomock
ModuleNotFoundError: No module named 'mongomock'
```

diff --git a/src/python/WMCore/Database/MongoDB.py b/src/python/WMCore/Database/MongoDB.py
index 43f3b4611..efd73f595 100644
--- a/src/python/WMCore/Database/MongoDB.py
+++ b/src/python/WMCore/Database/MongoDB.py
@@ -5,28 +5,24 @@ Description: Provides a wrapper class for MongoDB
 
 # futures
 from __future__ import division, print_function
-
 from builtins import str, object
 
+try:
+    import mongomock
+except ImportError:
+    # this library should only be required by unit tests
+    mongomock = None
+
 from pymongo import MongoClient, errors, IndexModel
-import mongomock
 
 
 class MongoDB(object):
     """
     A simple wrapper class for creating a connection to a MongoDB instance
     """
-    def __init__(self,
-                 database=None,
-                 server=None,
-                 port=None,
-                 replicaset=None,
-                 create=False,
-                 collections=None,
-                 testIndexes=False,
-                 logger=None,
-                 mockMongoDB=False,
-                 **kwargs):
+    def __init__(self, database=None, server=None, port=None, replicaset=None,
+                 create=False, collections=None, testIndexes=False,
+                 logger=None, mockMongoDB=False, **kwargs):
         """
         :databases:   A database Name to connect to
         :server:      The server url (see https://docs.mongodb.com/manual/reference/connection-string/)
@@ -44,6 +40,10 @@ class MongoDB(object):
         self.port = port # 8230
         self.logger = logger
         self.mockMongoDB = mockMongoDB
+        if mockMongoDB and mongomock is None:
+            msg = "You are trying to mock MongoDB, but you do not have mongomock in the python path."
+            self.logger.critical(msg)
+            raise ImportError(msg)
         try:
             if mockMongoDB:
                 self.client = mongomock.MongoClient()
['src/python/WMCore/Database/MongoDB.py']