MongoDB replicaset is now an option

In non-HA setup, we don't want MongoDB replicaset.
Let's put it as an option, enabled by default to keep backward
compatibility.
Merge pull request #648 from enovance/mongo-without-rs

MongoDB replicaset is now an option
N1KV VEM support to vSwitch

Bring n1kv_vem driver support to vSwitch configuration in Neutron
Compute & Network agent.

It use the n1kv_vem agent from puppet-neutron using default parameters
that could be replaced by custom datas.
Also add an unit test with a new context when running n1kv_vem agent.