Simplify database selection

Do not require every script that sources stackrc to also
source lib/databases.

* Move use_databases() to functions
* Set DATABASE_TYPE in stackrc
* Allow setting DATABASE_TYPE in localrc to work
  (use_database() essentially just sets DATABASE_TYPE at this stage
  so continuing to use it is equivalent)
* Validate DATABASE_TYPE in stack.sh.
* Change sudo to postgresql user to go through root to eliminate
  password prompt
* fix use_database error condition

Change-Id: Ibb080c76e6cd7c6eebbb641a894d54b1dde78ca6
diff --git a/lib/databases/postgresql b/lib/databases/postgresql
index 81989f2..ee24c8b 100644
--- a/lib/databases/postgresql
+++ b/lib/databases/postgresql
@@ -38,8 +38,8 @@
     start_service postgresql
 
     # If creating the role fails, chances are it already existed. Try to alter it.
-    sudo -u postgres -i psql -c "CREATE ROLE $DATABASE_USER WITH SUPERUSER LOGIN PASSWORD '$DATABASE_PASSWORD'" || \
-    sudo -u postgres -i psql -c "ALTER ROLE $DATABASE_USER WITH SUPERUSER LOGIN PASSWORD '$DATABASE_PASSWORD'"
+    sudo -u root sudo -u postgres -i psql -c "CREATE ROLE $DATABASE_USER WITH SUPERUSER LOGIN PASSWORD '$DATABASE_PASSWORD'" || \
+    sudo -u root sudo -u postgres -i psql -c "ALTER ROLE $DATABASE_USER WITH SUPERUSER LOGIN PASSWORD '$DATABASE_PASSWORD'"
 }
 
 function install_database_postgresql {