ML2: Use same port binding logic for DVR ports as non-DVR ports

ML2: Use same port binding logic for DVR ports as non-DVR ports

DVR ports are now bound using the same function,
Ml2Plugin._bind_port_if_needed(), that is used to bind non-DVR ports,
so that concurrent binding attempts are properly handled and mechanism
driver update_port_precommit() and update_port_postcommit() methods
are called. The Ml2Plugin._commit_dvr_port_binding() function is
eliminated, and the DvrPortContext class has been folded into the
PortContext class, reducing duplicated logic.

A followup patch will address the duplication of ML2 DB schema for DVR
and further reduce the duplicated and special-case port binding logic
supporting DVR.

Closes-Bug: 1415526
Closes-Bug: 1416783
Partial-Bug: 1367391

Change-Id: Ic32241297c5f8c67dc77d0af836b1cc0a5df988a