test_list_endpoints_for_token fails when heat plugin is installed

1. Problem
In the system where heat plugin is installed, running tempest identity testcase test_list_endpoints_for_token fails with
Traceback (most recent call last):
  File "/home/tempest/api/identity/admin/v2/test_tokens.py", line 135, in test_list_endpoints_for_token
    self.assertIn(service, service_names)
  File "/usr/lib/python2.7/site-packages/testtools/testcase.py", line 355, in assertIn
    self.assertThat(haystack, Contains(needle), message)
  File "/usr/lib/python2.7/site-packages/testtools/testcase.py", line 434, in assertThat
    raise mismatch_error
MismatchError: 'heat_plugin' not in [u'glance', u'ceilometer', u'heat-cfn', u'placement', u'panko', u'cinderv2', u'neutron', u'novav3', u'aodh', u'watcher', u'barbican', u'pimOps', u'cinder', u'swift', u'keystone', u'nova', u'keystonev3', u'vimOps', u'cinderv3', u'ironic', u'camelliav1', u'heat', u'magnum']

2. Analysis
in heat_plugin's config.py, there is
ServiceAvailableGroup = [
    cfg.BoolOpt("heat_plugin",
                default=True,
                help="Whether or not heat is expected to be available"),
]

so Tempest will get CONF.service_available.heat_plugin=True, but obviously we haven't any endpoints named heat_plugin in the system, so the testcase will fail.

3. Solution
It seems CONF.service_available.heat_plugin is not used anywhere, so we can set its default value to False to avoid causing tempest identity testcase failing(which is hard for someone not familiar with heat plugin to debug what's wrong). (We don't remove it in case it has any future usage)