Preserve the overcloud rather than deleting it.

Users can delete it if they want, but updating should be the default,
because who wants to throw away a cloud on every run!

Change-Id: I3486640fc29d12f5ffdb50d6873dcb8272376ef7
Permit setting the overcloud image update policy.

This is needed to preserve /mnt when rebuilding nodes. In principle we
can do stateful updates with rolling heat stack updates and live
migrations of tenants, cinder data and swift data - but that is a lot
of overhead to use every time - this is just much quicker and is the
path we have agreed on for getting out of the quagmire we're in where
no-one can use us for production image based updates.

Change-Id: I0efa8f52864f49ccdb885f6f655c732c951b3f7a