Skip when force_tenant_isolation=true and no admin credentials

AbsoluteLimitsTests requires force_tenant_isolation=True,
and if admin credentials are not present, the testcase will
fail instead of being skipped.

This is to let AbsoluteLimitsTests inherit from BaseVolumeAdminTest
so it can skip if admin credentials are not present.

Change-Id: Id71a705cf9b1dd0c0d41a2fb45ab77c95430a123
Closes-Bug: #1671256
diff --git a/tempest/api/volume/test_volume_absolute_limits.py b/tempest/api/volume/test_volume_absolute_limits.py
index 836e489..870b9f0 100644
--- a/tempest/api/volume/test_volume_absolute_limits.py
+++ b/tempest/api/volume/test_volume_absolute_limits.py
@@ -21,7 +21,10 @@
 CONF = config.CONF
 
 
-class AbsoluteLimitsTests(base.BaseVolumeTest):
+# NOTE(zhufl): This inherits from BaseVolumeAdminTest because
+# it requires force_tenant_isolation=True, which need admin
+# credentials to create non-admin users for the tests.
+class AbsoluteLimitsTests(base.BaseVolumeAdminTest):
 
     # avoid existing volumes of pre-defined tenant
     force_tenant_isolation = True