Fix for Cisco plugin physical device config error

This patch fixes a bug with the Cisco plugin where subsequent vlan id's
would overwrite the existing trunking config on a switch interface.

Change-Id: I5e97c00062f687838e60e5dc9470f4a62289676b
Fixes: Bug #1161946
Merge "Fix for Cisco plugin physical device config error" into stable/grizzly