Don't run swap_volume tests in Tempest if cells v1 is enabled

Cells v1 apparently doesn't support the swap volume API which
was recently enabled for testing in change:

92575baa6b010ec09fea3e715030da8be61c0c64

Rather than revert that change, we should just handle the cells
v1 case and not enable that test in that environment.

Change-Id: I80f52e8299641098d90d3c374a80770fc45b8122
Closes-Bug: #1660511
diff --git a/lib/tempest b/lib/tempest
index 050ac38..7cafadb 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -516,8 +516,10 @@
             iniset $TEMPEST_CONFIG compute-feature-enabled shelve False
             iniset $TEMPEST_CONFIG compute-feature-enabled snapshot False
             iniset $TEMPEST_CONFIG compute-feature-enabled suspend False
+        elif ! is_service_enabled n-cell; then
+            # cells v1 does not support swapping volumes
+            iniset $TEMPEST_CONFIG compute-feature-enabled swap_volume True
         fi
-        iniset $TEMPEST_CONFIG compute-feature-enabled swap_volume True
     fi
 
     # ``service_available``