Fix Cisco nexus plugin failure for first VLAN on phy interface

Fixes bug 1174852

This fix changes the command being sent to the Nexus switch for
enabling a VLAN on an interface such that the 'add' keyword is
included only when there are prior VLANs already enabled on that
interface.

(cherry picked from commit - 92ef293d02a7499850bc3becda413d6be22ae06c)

Change-Id: I900e0cac9f88d38b5276a65bdfa7457aed466199
Merge "Fix Cisco nexus plugin failure for first VLAN on phy interface" into stable/grizzly