Use volume_type option in tempest.conf in compute api test

A new configuration option volume_type was added to tempest.conf
to allow tempest users to select volume type during volume creation.

In this patchset, this volume_type value is used if configured for
compute api volume creation.

Change-Id: I031094ab196268dc2c20b38be4864f59639358a7
diff --git a/tempest/api/compute/base.py b/tempest/api/compute/base.py
index 7900b77..bee4716 100644
--- a/tempest/api/compute/base.py
+++ b/tempest/api/compute/base.py
@@ -515,6 +515,12 @@
             kwargs['display_name'] = vol_name
         if image_ref is not None:
             kwargs['imageRef'] = image_ref
+        if CONF.volume.volume_type and 'volume_type' not in kwargs:
+            # If volume_type is not provided in config then no need to
+            # add a volume type and
+            # if volume_type has already been added by child class then
+            # no need to override.
+            kwargs['volume_type'] = CONF.volume.volume_type
         if CONF.compute.compute_volume_common_az:
             kwargs.setdefault('availability_zone',
                               CONF.compute.compute_volume_common_az)