Add test-config local.conf processing

We need a local.conf processing phase after every run phase which lets
us override config options after that point. We didn't explicitly
support this for test-config before, which broke some CI systems when
we moved tempest to use this later phase.

Closes-Bug: #1646391

Change-Id: I7d693afa19acf3e8231e84e45b7a868628ebdbc0

Author: sdague

stack.sh

@@ -1407,6 +1407,9 @@ fi
 # Phase: test-config
 run_phase stack test-config
 
+# Apply late configuration from ``local.conf`` if it exists for layer 2 services
+# Phase: test-config
+merge_config_group $TOP_DIR/local.conf test-config
 
 # Fin
 # ===