N1KV support to Virtual Networking

Plugin Side: cloud::network
===========
- Add a flexibility to modify default ML2 plugin.
- Add non-ML2 Cisco plugin support.

Note: 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.

Agent Side: cloud::network::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.
- ensure nexus1000v package is installed
- Also add an unit test with a new context when running n1kv_vem agent.

Also update the README.
You remove the `ml2_enabled` parameter, please deprecate this parameter instead! (using a deprecation message as others params).