Decouple boot-stack from non-Heat configuration.

At the moment boot-stack assumes it is being booted without Heat or
any other cloud infrastructure. This is only correct for the seed node
- everything after that is booted into an environment with working
Heat, Neutron etc.

By splitting it out we make boot-stack more suitable for bootstrapping
the undercloud and overcloud clouds.

Change-Id: I6051da5ee708b866d90e095c51e105da919bfeef