ML2 Cisco Nexus MD: Create pre/post DB event handlers

ML2 Cisco Nexus MD: Create pre/post DB event handlers

Split ML2 cisco nexus event handers for update and delete
into precommit (called during DB transactions) and postcommit
(called after DB transactions) methods.

Also fixes some unit tests that were incorrectly accessing
context managers without using the "with" statement.

Closes-Bug: #1241098
Change-Id: I59b046342706230222c1be39d13a455ca5a884ea