Fix mongodb_user.py version detection logic (#3162)

Fix mongodb_user.py version detection logic for mongo srv 2.6 and mongo driver 2.7. The wrong variable was used for detecting the mongo driver version. This fix resolves the error "(Note: you must use pymongo 2.7+ with MongoDB 2.6.. 2.6.11)" no matter what version of pymongo you had installed for mongodb 2.6.

lib/ansible/modules/extras/database/misc/mongodb_user.py

@@ -185,7 +185,7 @@ def check_compatibility(module, client):
     elif loose_srv_version >= LooseVersion('3.0') and loose_driver_version <= LooseVersion('2.8'):
         module.fail_json(msg=' (Note: you must use pymongo 2.8+ with MongoDB 3.0)')
 
-    elif loose_srv_version >= LooseVersion('2.6') and loose_srv_version <= LooseVersion('2.7'):
+    elif loose_srv_version >= LooseVersion('2.6') and loose_driver_version <= LooseVersion('2.7'):
         module.fail_json(msg=' (Note: you must use pymongo 2.7+ with MongoDB 2.6)')
 
     elif LooseVersion(PyMongoVersion) <= LooseVersion('2.5'):