Allow skipping manila tempest tests.

This commit allows to skip manila tempest tests when manila is set to
false in tempest.CONF service_available. This would allow to disable
manila testing in an environment where all the plugins are installed
(i.e. a package-based installation).

Change-Id: I75dd2fd86df7500ad0bc4f633ed39f7595e9dc8c
diff --git a/manila_tempest_tests/tests/api/base.py b/manila_tempest_tests/tests/api/base.py
index 7bb727d..d5d1175 100644
--- a/manila_tempest_tests/tests/api/base.py
+++ b/manila_tempest_tests/tests/api/base.py
@@ -221,6 +221,12 @@
         return client
 
     @classmethod
+    def skip_checks(cls):
+        super(BaseSharesTest, cls).skip_checks()
+        if not CONF.service_available.manila:
+            raise cls.skipException("Manila support is required")
+
+    @classmethod
     def verify_nonempty(cls, *args):
         if not all(args):
             msg = "Missing API credentials in configuration."