Segregate the VSM calls from database calls in N1kv plugin

Rearrange the code in such a way that the calls to the VSM are
outside the database session. This avoids DB locks on n1kv tables.

Change-Id: Ic0f0d816cca06a3b03fdd9b58ee7599ddf65473e
Closes-Bug: 1290646

Conflicts:

	neutron/plugins/cisco/n1kv/n1kv_neutron_plugin.py