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
c1b486a520