Allow DATABASE_TYPE to be specified in localrc

The use of DATABASE_TYPE without forcing it into ENABLED_SERVICES
seems to be the only way to set up a multi-host devstack installation.

Only deprecate use_database.
Changes functionality of use_database to be equivalent to specifying DATABASE_TYPE
This means that people with localrc's that only specify use_database used to have
a database service brought up, but now will not until they add it to ENABLED_SERVICES.

Change-Id: I4c6e8ee31f298004338ca46bfc330a3e3af288aa
Fixes: bug 1136028
Merge "Allow DATABASE_TYPE to be specified in localrc"