DB Mappings for NSX security groups

This patch introduces DB mappings between neutron security
groups and NSX security profiles, thus not requiring anymore
the Neutron router ID to be equal to the NSX one.

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

Related to blueprint nvp-async-backend-communication

Change-Id: I3b28d535c93cd2bfc776aabe0d99be18fce4454d