use same password and username env vars for all types

Dockerfile

@@ -23,10 +23,8 @@ ENV STORAGE_TYPE=memcached \
     STORAGE_DB=2 \
     STORAGE_AWS_BUCKET= \
     STORAGE_AWS_REGION= \
-    STORAGE_PG_PASSWORD= \
-    STORAGE_PG_USERNAME= \
-    STORAGE_RETHINK_USER= \
-    STORAGE_RETHINK_PASSWORD= \
+    STORAGE_USENAMER= \
+    STORAGE_PASSWORD= \
     STORAGE_FILEPATH= 
 
 ENV LOGGING_LEVEL=verbose \