Enable Ceilometer to support mongodb replication set

At present, ceilometer does not support mongodb replication set.
As a result, when the primary mongod node is down, ceilometer can
not write or read meters.

Use pymongo.Connection object's replicaSet param and allowing to connect
with a host seed list will let the mongodb connection support replication set.

Change-Id: I8404ca5b08b6e73366161c07b3815f75bed7e0eb
Fixes: Bug 1188649

Conflicts:
	ceilometer/storage/impl_mongodb.py