Enable quantum agents to work with global cfg.CONF

Enable quantum agents to work with global cfg.CONF

This implements blueprint global-config-support

A common configuration parse function is used by the service and the agent.

In the case of the service, when loading the plugin, the configuration file
will be appended to the existing cfg.CONF (this was originally created when
quantum.conf was loaded).

In the case of the agent a global cfg.CONF will be created.

This task is a prerequisite for the RPC integration into Quantum.

Change-Id: I24ef4a804578502bcce484f44dc55c8fe2aa913e