Add image_username and image_password to charms

This commit will allow us to pull images for OSM components from a
private docker registry, protected with username and password

Change-Id: I854028d6abbb1cae07c61692ba73d08825763212
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/installers/charm/pla/config.yaml b/installers/charm/pla/config.yaml
index e0aeca4..f3318fc 100644
--- a/installers/charm/pla/config.yaml
+++ b/installers/charm/pla/config.yaml
@@ -20,6 +20,14 @@
     description: Docker image name
     type: string
     default: opensourcemano/pla:8
+  image_username:
+    description: Docker repository username
+    type: string
+    default: ""
+  image_password:
+    description: Docker repository password
+    type: string
+    default: ""
   port:
     description: Port number
     type: int