Refactored impl_mongodb to use full connection url

Host parameter can be a full URI see:
http://docs.mongodb.org/manual/reference/connection-string/

This allows to set replicaSet config and other options in the connection
URL without the need to have additional config values

Replaced deprecated pymongo.Connection with pymongo.MongoClient

Change-Id: I1aea1d8a48dc1bec717e7d1fbe256e2887e27039
Merge "Refactored impl_mongodb to use full connection url"