Implements the blueprint use-common-cfg for the quantum service.

Implements the blueprint use-common-cfg for the quantum service.
More specifically uses global CONF for the quantum.conf file.

Added support for the RYU plugin (similar to ovs and lb,
which use non-global conf for plugins)

patch 27: clean up find_config_file
patch 28: for config file use old paths (plugin unit tests)
          this hopefully will be replaced when we move to common
          config file
patch 30: rebase and merge (utils.py and policy.py)

Change-Id: Ic0bf5bdd44f24a557240f7afe4e070dee448c63c