Add DB mappings with NSX logical switches

This patch introduces DB mappings between neutron network and NSX
logical switches, thus not requiring anymore the Neutron network
ID to be equal to the NSX one.

This change is necessary for enabling asynchronous operations in
the NSX plugin.

This patch also performs NVP/NSX renaming where appropriate.

Related to blueprint nvp-async-backend-communication
Related to blueprint nicira-plugin-renaming

Change-Id: Ic4b65730ee5f9ca5f76f89eb2528709c9cc7973e
Merge "Add DB mappings with NSX logical switches"