Improve unit test coverage for Cisco plugin common code

Closes-Bug: #1190615

This fix improves the unit test coverage for the Cisco Nexus plugin
common modules (directory neutron/plugins/cisco/common) from:
--- cisco_credentials_v2.py 82%
--- cisco_faults.py 0%
--- config.py 84%
To:
--- cisco_credentials_v2.py 100%
--- cisco_faults.py 76%
--- config.py 100%

Change-Id: Id1ccfee1317309039fab83f622ced58efc3b2ae7
Merge "Improve unit test coverage for Cisco plugin common code"