This patch adds a new DefaultPasswords parameter to
composable services. This is needed to help provide
access to top level password resources that overcloud.yaml
currently manages (passwords for Rabbit, Mysql, etc.).
Moving the RandomString resources into composable services
would cause them to regenerate within the stack. With this
approach we can leave them where they are while we deprecate
the top level mechanism and move the code that uses the
passwords into the composable services.
Change-Id: I4f21603c58a169a093962594e860933306879e3f
3b62761d2f