Fix connecting to a list of hosts

[jtharpla]

Fixes a regression caused by #1151 where host can no longer be a list when calling connect() as this triggers the following error:

    mongoengine/connection.py line 61 in register_connection
      if conn_host.startswith('mongomock://'):
   AttributeError: 'list' object has no attribute 'startswith'

Adds a new unit test to cover this possibility and follows the same approach used by pymongo.MongoClient, which is designed to accept host as either a list or a string.

[lafrech]

Thanks.