Add possibility to not compile ovs and ovn if that's disabled
Add possibility to not compile ovs and ovn if that's disabled

There is flag Q_BUILD_OVS_FROM_GIT which can be used to not compile
ovs from source.
But this wasn't respected in the ovn_agent's module in install_ovn
function which was always installing from source ovn and ovs.
We need to disable that e.g. on grenade jobs when new version is
installed.

Change-Id: I7d3f92365e880191dcfe7c618a6f79d5f741144f

nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack