Decouple AttachVolumeShelveTestJSON from AttachVolumeTestJSON

Since AttachVolumeShelveTestJSON was extending AttachVolumeTestJSON
to re-use common methods and setup, it was also running the tests
from the base class which have nothing to do with shelve/unshelve
scenarios, and adding around 3 minutes to each Tempest run.

This decouples the classes that run the actual tests by moving
the common code and setup into a common base class which does not
have any tests.

The max_microversion restriction is removed from AttachVolumeTestJSON
because there is nothing microversion-specific about the tests in
that class and it was added with AttachVolumeShelveTestJSON when it
extended AttachVolumeTestJSON.

Change-Id: I8cb9cbb162361cbf4adbea11b8ffaa7f7c45485f
diff --git a/tempest/api/compute/volumes/test_attach_volume.py b/tempest/api/compute/volumes/test_attach_volume.py
index 297e8a8..e6184b7 100644
--- a/tempest/api/compute/volumes/test_attach_volume.py
+++ b/tempest/api/compute/volumes/test_attach_volume.py
@@ -23,12 +23,12 @@
 CONF = config.CONF
 
 
-class AttachVolumeTestJSON(base.BaseV2ComputeTest):
-    max_microversion = '2.19'
+class BaseAttachVolumeTest(base.BaseV2ComputeTest):
+    """Base class for the attach volume tests in this module."""
 
     @classmethod
     def skip_checks(cls):
-        super(AttachVolumeTestJSON, cls).skip_checks()
+        super(BaseAttachVolumeTest, cls).skip_checks()
         if not CONF.service_available.cinder:
             skip_msg = ("%s skipped as Cinder is not available" % cls.__name__)
             raise cls.skipException(skip_msg)
@@ -36,11 +36,11 @@
     @classmethod
     def setup_credentials(cls):
         cls.prepare_instance_network()
-        super(AttachVolumeTestJSON, cls).setup_credentials()
+        super(BaseAttachVolumeTest, cls).setup_credentials()
 
     @classmethod
     def resource_setup(cls):
-        super(AttachVolumeTestJSON, cls).resource_setup()
+        super(BaseAttachVolumeTest, cls).resource_setup()
         cls.device = CONF.compute.volume_device_name
 
     def _create_server(self):
@@ -58,6 +58,9 @@
             server['id'])['addresses']
         return server, validation_resources
 
+
+class AttachVolumeTestJSON(BaseAttachVolumeTest):
+
     @decorators.idempotent_id('52e9045a-e90d-4c0d-9087-79d657faffff')
     def test_attach_detach_volume(self):
         # Stop and Start a server with an attached volume, ensuring that
@@ -149,7 +152,7 @@
                 self.volumes_client, attachment['volumeId'], 'available')
 
 
-class AttachVolumeShelveTestJSON(AttachVolumeTestJSON):
+class AttachVolumeShelveTestJSON(BaseAttachVolumeTest):
     """Testing volume with shelved instance.
 
     This test checks the attaching and detaching volumes from