Make tempest L3 capable plugin aware

With this patch, the public network config in tempest.conf will be
done for the plugins that support L3.

This is required to enable third-party CI for stable/havana

Conflicts:

	lib/neutron
	lib/tempest

Change-Id: I820fe300fac45ff92d1281ff0c43ebc137783210
(cherry picked from commit afbc631cb8c89316bbecbf0f2c601103304e1994)
Merge "Make tempest L3 capable plugin aware" into stable/havana