Deprecate DATABASE_TYPE and use_database

Select a database by adding it to ENABLED_SERVICE like the other
services.  This greatly simplifies using the lib/* functions in
places other than stack.sh

Backward-compatibility is maintained or now (into havana at least).

Change-Id: I967e44603b4d69d5d70e1a75a9938172ca434025
diff --git a/stackrc b/stackrc
index 789fc82..f53aeec 100644
--- a/stackrc
+++ b/stackrc
@@ -9,9 +9,6 @@
 # Destination for working data
 DATA_DIR=${DEST}/data
 
-# Select the default database
-DATABASE_TYPE=mysql
-
 # Determine stack user
 if [[ $EUID -eq 0 ]]; then
     STACK_USER=stack
@@ -24,7 +21,7 @@
 # ``disable_service`` functions in ``localrc``.
 # For example, to enable Swift add this to ``localrc``:
 # enable_service swift
-ENABLED_SERVICES=g-api,g-reg,key,n-api,n-crt,n-obj,n-cpu,n-net,n-cond,cinder,c-sch,c-api,c-vol,n-sch,n-novnc,n-xvnc,n-cauth,horizon,rabbit,tempest,$DATABASE_TYPE
+ENABLED_SERVICES=g-api,g-reg,key,n-api,n-crt,n-obj,n-cpu,n-net,n-cond,cinder,c-sch,c-api,c-vol,n-sch,n-novnc,n-xvnc,n-cauth,horizon,rabbit,tempest,mysql
 
 # Set the default Nova APIs to enable
 NOVA_ENABLED_APIS=ec2,osapi_compute,metadata