Cisco plugin db code cleanup, part II

Remove an unused table.
Prefix Cisco tablenames with cisco_.
Make full use of neutron model base class.
DRY out the nexusport binding queries.
Follow coding guidelines for imports.
Improve cisco/db/network_db_v2.py test coverage.
Improve cisco/db/nexus_db_v2.py test coverage.

Change-Id: I8ea110de37552176f2d9bcbff4ca3e0b65dfacdc
Closes-bug: #1202687
Partial-bug: #1190619
Merge "Cisco plugin db code cleanup, part II"