Fix ML2 Plugin binding:profile update

The current fix changes the logic for binding:profile
update. The binding:profile should be considered as changed
once it is present in the port attributes and differs from
existing binding:profile. The specified binding:profile with
None value should be treated as request to clear binding:profile.

Change-Id: Ibda9a1beec697fbee5be0ee379349035c3626509
Closes-Bug: 1338202
Merge "Fix ML2 Plugin binding:profile update"