Connection to replica sets

[kcaylor]

The connection assumes a single host and does not accommodate the possibility of a replica set. The easiest fix would be to allow for the use of a MONGO_URI when setting up the connection, which - if set - would be used to make the connection rather than building the connection using the MONGO_HOST, and MONGO_PORT variables. If MONGO_URI is not set, then the behavior would remain as in the current implementation. Thoughts?

[Alain1405]

This is a must for deployment on Heroku, as the main mongodb addons only export database credentials in a MONGO_URI format.
Isn't eve-mongoengine importing flask-pymongo? It supports MONGO_URI