Allow None for binding:profile attribute

Allow None for binding:profile attribute

We need to pass None in binding:profile to allow an administrator
to clear binding:profile attribute.

Closes-Bug: #1220011

Adds dedicated unit tests to the plugins which uses binding:profile
attribute (Mellanox and NEC plugins at now).

This commit also adds common unit tests for binding:profile to
the common PortBindingTestCase class.
- create_port with binding:profile whose value is None or {}
- update_port with binding:profile whose value is None or {}
- Reject binding:profile from non-admin user

Note that _make_port() in BigSwitch plugin test is updated
to allow passing arg_list() from the base test class.

Fix a bug in NEC plugin that 500 is returned when putting
binding:profile None to a port whose binding:profile is
already None (Closes-Bug: #1220720)

Change-Id: I146afe961cd445a023adc7233588d8034fdb8437