Only disable Neutron tests when our Grenade is running

Unconditionally disabling it in code disables neutron for everyone
importing our plugin.

Change-Id: I468af14f42b6d9227179e921f5a3ccea2cae0d66
Partial-Bug: #1621791
diff --git a/ironic_tempest_plugin/plugin.py b/ironic_tempest_plugin/plugin.py
index 32a6d37..218f100 100644
--- a/ironic_tempest_plugin/plugin.py
+++ b/ironic_tempest_plugin/plugin.py
@@ -33,9 +33,10 @@
         conf.register_group(config.baremetal_introspection_group)
         conf.register_opts(config.BaremetalIntrospectionGroup,
                            group="baremetal_introspection")
-        # FIXME(dtantsur): pretend like Neutron does not exist due to random
-        # failures, see https://bugs.launchpad.net/bugs/1621791.
-        cfg.CONF.set_override('neutron', False, 'service_available')
+        if os.path.exists('/tmp/ironic-inspector-grenade'):
+            # FIXME(dtantsur): pretend like Neutron does not exist due to
+            # random failures, see https://bugs.launchpad.net/bugs/1621791.
+            cfg.CONF.set_override('neutron', False, 'service_available')
 
     def get_opt_lists(self):
         return [