Fix heat service availability config option name

heat service config option name registered in
Tempest's service_available group is 'heat_plugin' which
is not consistent with all other service_available config
option.

This config option is used by other tempest plugin to test the heat.
One good example is congress-tempest-plugin which has heat tests[1]
and those test are being skipped based on this config option.

But setting of this config option in congress devstack plugin
is done based on service name (heat not heat_plugin)[2]. and same name
is used on devstack side also[3].

We had same issue on aodh side also which is being fixed in
- https://review.openstack.org/#/c/536741/

This patch fixes this issue in heat-tempest-plugin also by
renaming the config option to 'heat' and keep the old name support also
with deprecation warning.
Related-Bug: #1743688

[1] http://git.openstack.org/cgit/openstack/congress-tempest-plugin/tree/congress_tempest_plugin/tests/scenario/congress_datasources/test_heat.py#n32
[2] https://github.com/openstack/congress/blob/df0f555f3166344b726fbfd2356e3a7d897cf228/devstack/plugin.sh#L87
[3] https://github.com/openstack-dev/devstack/blob/0c6208c6a07b0616b5982b6e9e7c8216397d49e3/lib/tempest#L550

Change-Id: Ibe080d08a854db1a3e7246ef60a3a14efdc2f40f

heat_tempest_plugin/config.py

@@ -16,9 +16,12 @@
                                        title="Available OpenStack Services")
 
 ServiceAvailableGroup = [
-    cfg.BoolOpt("heat_plugin",
+    cfg.BoolOpt("heat",
                 default=True,
-                help="Whether or not heat is expected to be available"),
+                help="Whether or not heat is expected to be available",
+                deprecated_opts=[cfg.DeprecatedOpt(
+                    'heat_plugin',
+                    group='service_available')]),
 ]
 
 heat_group = cfg.OptGroup(name="heat_plugin",