ML2: Remove validate_port_binding() and unbind_port()

The API implemented by ML2 mechanism drivers included three methods
related to port binding that were called within DB transactions, but
that could potentially involve communication with controllers or
devices that should not be done within transactions. A subsequent
patch will move the calls to bind_port() outside of tranactions. This
patch eliminates the other two methods from the MechanismDriver API.

The validate_port_binding() method was previously called on the bound
mechanism driver to check whether an existing binding was still valid,
so that the port could be rebound if something changed. But since nova
has no way to handle changes to binding:vif_type or
binding:vif_details after a port is initially plugged, this turned out
not to be useful, so the method has been removed from the
MechanismDriver API. Now, once a port is successfully bound, the
binding remains until the port is deleted or any of it's
binding:host_id, binding:vnic_type, or binding:profile attribute
values are changed.

The unbind_port() method was previously called on the bound mechanism
driver as an existing binding was removed. This method was not used by
any existing mechanism drivers, and was redundant with the
update_port_precommit() and update_port_postcommit() methods that are
called on all mechanism drivers when an existing binding is removed,
so this method has also been removed from the driver API.

Eliminating the unbind_port() call allows the binding details to be
made available via the PortContext in delete_port_postcommit() calls,
completing the resolution of bug 1276395.

Closes-bug: 1276395
Partial-bug: 1276391
Change-Id: I70fb65b478373c4f07f5273baa097fc50e5ba2ef
Merge "ML2: Remove validate_port_binding() and unbind_port()"