Cisco plugin should check for switch - vlan bindings

This commit fixes the issue where the Cisco plugin tries to create a
vlan twice on a switch if the first create is not bound to a port.
Also fixes an issue where the plugin tried to untrunk vlans from
a port for SVI interfaces.

Change-Id: Ifb13eb65d667367cffe120e1761d34f09b6d356d
Fixes: Bug #1233300