make odl_features default to empty feature set
make odl_features default to empty feature set

Tests that do not need any features turned on
should not be forced to initialize odl_features
by using OpendaylightFeaturesFixture. After this
change all features will be implicitly off.

Change-Id: Iff40030dc84e4662cf33d3a9a875e3e895fb6627

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl