ML2 Cisco Nexus MD: Fix UT to send one create vlan message

With the commit of https://review.openstack.org/#/c/113009,
test_nexus_add_trunk needs to have the device_id set to
a unique value. Combining test_nexus_add_trunk and
test_nexus_enable_vlan_cmd to reduce duplicate code,
which fixes the original issue.

Change-Id: I4191a6880ec0f43365aacb638a92a0dd3e1e80aa
Closes-Bug: 1364976
Merge "ML2 Cisco Nexus MD: Fix UT to send one create vlan message"