With Cisco/Nexus plugin, migration is not fully supported. Logic to detect
port binding change needs to be added in update_port(), and provisioning of
nexus switch(es) should be done accordingly
added test code for update_port() in the model layer and the db layer
Closes-Bug: #1229217
Change-Id: I2bd76030711c9d15462e91da9e4c0836a424834f