Move to use "get_password" instead of "{user,service,db}_password"

The user_password, service_password and db_password functions are redundant
since they simply call "secret". Creates a get_password function that
will accept a "type" of db, service or user.

All instances of these calls have been changed to call get_password.

Interim commit that keeps the service,db and user functions in order to
merge other changes.

Change-Id: Iba4a611b387d0975e8a23cc758d2ac7dec8210ad
Partial-Bug: #1195915