Common config support. (blueprint cisco-single-config plus N1K support.)

Implementation Overview: All the config values contained in the various files in
etc/quantum/plugins/cisco will be unified into a single file
etc/quantum/plugins/cisco/cisco_plugin.ini. The plugins needs to be modified to
read from a single file instead of multiple.
Added quantum.openstack.common.cfg support.
Added N1k single config support.

Change-Id: I71d487f25056cf7854587dd5b7ceb35f986e5069
Merge branch 'n1k-grizzly' of https://github.com/CiscoSystems/quantum into n1k-grizzly