Compute: Exec puppet after all configuration

Compute: Exec puppet after all configuration

This patch adds a new ComputeNodesPostDeployment resource
which can be used along with the environment file to
specify a nested stack which is guaranteed to execute
after all the Compute config deployments have executed.

This is really useful for Puppet in that Heat actually
controls where puppet executes in the deployment
process and we want to ensure puppet runs after
all hiera configuration data has be deployed to
the nodes. With the previous approach some of the
data would be there, but allNodes data would not be
guaranteed to be there in time.

As os-apply-config (tripleo-image-elements) have their
ordering controlled within the elements themselves an empty stubbed
in nested stack has been added so that we don't break that
implementation.

Change-Id: I80bccd692e45393f8250607073d1fe7beb0d7396