Update neutron_available config option.

Tempest change I5ee9ec816845de483fe88d76d1bb047e7bb1af7e changed
the behavior of the neutron_available config option. This commit
updates devstack to use it's new name and group.

Change-Id: I7d5074209fe81f6100f380512d7702fbc8e252ac
Merge branch 'master' of github.com:openstack-dev/devstack