Currently devstack assumes that the network used for ssh
validation is the private network. This patch adds a hook that
sets the network used for ssh validation based on whether or not
provider networking is being used. It also moves the function
'is_provider_network' into functions-common as it will now be
used by both tempest and neutron.
Change-Id: I265c9e26c9bfb18b7e201f27d8912b8bec235872