Introduce SERVICES_FOR_TEMPEST variable for localrc

This variable overrides the `ENABLED_SERVICES` global variable only for
the `configure_tempest()` function from `lib/tempest`. If the
`SERVICES_FOR_TEMPEST` variable is not defined then `ENABLED_SERVICES`
is used as it had used it before.

This is useful for cases where Tempest is executed from a remote node.

Change-Id: Ic62e48f2f1eb861ec64f51e03353868076cbcc04
diff --git a/lib/tempest b/lib/tempest
index eeeef67..b8f9915 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -197,6 +197,8 @@
         pip_install_gr testrepository
     fi
 
+    local ENABLED_SERVICES=${SERVICES_FOR_TEMPEST:=$ENABLED_SERVICES}
+
     local image_lines
     local images
     local num_images