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
Set service_available config options in tempest

This commit loops over the list of services that tempest has config
options for in the service_available group and checks if the service
is enabled. Devstack will then set whether or not the service is
configured in tempest.conf.

Change-Id: Ib845d3e098fd3f45c8c26f5696af14cca1534e01