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.
Merge pull request #635 from enovance/n1kv-neutron

Cisco Support for Virtual Networking