Allow to decide between Nova-network and Neutron

Adds a heat.conf option to set the OpenStack component responsible for
networking, and makes heat-engine auto-discover the networking service
on start up if such option is not set.
Also adds a convenience method to Resource class to let resources decide
what networking service to use.

Implements blueprint discover-networking-service

Change-Id: If7121089068cc2d2774bedb73e4e252b520eb5b3
Merge "Allow to decide between Nova-network and Neutron"