Disable compute interface attach Tempest tests if using cells

Cells doesn't support the os-attach-interfaces API so disable those
tests in Tempest if running with Cells.

Change-Id: I5c7884407868eae70ea125f3f893c73214c04c75

Author: Matt Riedemann

lib/tempest

@@ -388,6 +388,8 @@ function configure_tempest {
     if is_service_enabled n-cell; then
         # Cells doesn't support shelving/unshelving
         iniset $TEMPEST_CONFIG compute-feature-enabled shelve False
+        # Cells doesn't support hot-plugging virtual interfaces.
+        iniset $TEMPEST_CONFIG compute-feature-enabled interface_attach False
     fi
 
     # Network