Avoid re-wiring ports unnecessarily

Avoid re-wiring ports unnecessarily

In the majority of cases a port_update notification pertains
a change in the properties affecting port filter, and does
not affect port wiring, ie: the local vlan tag.

This patch simply avoids doing port wiring/unwiring if the
local vlan tag did not change.
The extra overhead for the ovs-db get operation is offset
by the fact that get commands are generally faster than
set commands, and by avoiding executing the ovs-ofctl operation.

Partial-Bug: #1253993
Partially implements blueprint: neutron-tempest-parallel

Change-Id: Ia0bd2dc4e5a2634a4c863ff32ccc5cabe8e21337