N1KV support to Virtual Networking

Plugin Side
===========
Add a flexibility to modify default ML2 plugin to use Cisco plugin.
For now, the configuration is for Cisco Nexus with
neutron.plugins.cisco.network_plugin.PluginV2 core plugin &
neutron.plugins.cisco.n1kv.n1kv_neutron_plugin.N1kvNeutronPluginV2
vswitch plugin.
For now, it considers that n1kv vem package is already installed.
It may evolve in the future.

Agent Side
==========
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.

TODO
====

- Manage Cisco VSM.
- Manage Cisco Networking policies in Plugin configuration
- Unit tests