Enable agents and plugins to use the same configuration file.

Enable agents and plugins to use the same configuration file.

The change also implements blueprint agent-logging (this
was due to the fact that the existing logging in the agents
did not work with the change)

Devstack support for the above is in
https://review.openstack.org/10267

Notes:
- The service and agent can be run with multiple configuration
  files by using the CLI parameters --config-file <filename>
- The agent now makes use of the logging and debugging flags
  defined in quantum.conf. This follows the standard common
  configuration command line rules.

Change-Id: I3bd7701526a950c360d7c2cf14b31045010b46ca