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

lib/database

@@ -62,15 +62,6 @@ function initialize_database_backends {
     return 0
 }
 
-# Set the database backend to use
-#  $1 The name of the database backend to use (mysql, postgresql, ...)
-function use_database {
-    use_exclusive_service DATABASE_BACKENDS DATABASE_TYPE $1 && return 0
-    ret=$?
-    echo "Invalid database '$1'"
-    return $ret
-}
-
 # Recreate a given database
 #  $1 The name of the database
 #  $2 The character set/encoding of the database