Update default db username to keystone

The current default db username for keystone is
keystone_admin. This is inconsistent with the default
db username for every other service which use the
same name as the name of the service.

The documented installation instruction for keystone
also use keystone as the database user.

This commit updates the default to use keystone instead of
keyston_admin.

Change-Id: I1cfaf3fbbc691ff9dbef415b69492f9f965dc113
Merge "Update default db username to keystone"