Pin PLA image tag to "8"

This needs to be reverted when PLA is correctly published to DockerHub

Change-Id: I1aa509288ae49233f110e5951c491f3598be8b05
Signed-off-by: David Garcia <david.garcia@canonical.com>

installers/charmed_install.sh

@@ -392,7 +392,7 @@ applications:
       image: ${REGISTRY_URL}opensourcemano/pol:$TAG ${REGISTRY_CREDENTIALS}
   pla:
     options:
-      image: ${REGISTRY_URL}opensourcemano/pla:$TAG ${REGISTRY_CREDENTIALS}
+      image: ${REGISTRY_URL}opensourcemano/pla:8 ${REGISTRY_CREDENTIALS}
   ng-ui:
     resources:
       image: /tmp/ng_ui_registry.yaml