NSX: propagate network name updates to backend

This patch updates the logical switch name on the NSX backend
if a new network name is provided in the update request.

This patches also fixes tag management for update_lswitch in
neutron.plugins.vmware.nsxlib.switch and add relevant unit
tests.

Closes-Bug: 1276128
Change-Id: Ic8921c059012a875006701ac10d2dcae97f30253
Merge "NSX: propagate network name updates to backend"