Add "AllNodes" ExtraConfig interface

Add "AllNodes" ExtraConfig interface

Adds hook to enable additional "AllNodes" config to be performed prior
to applying puppet - this is useful when you need to build
configuration data which requires knowledge of all nodes in a cluster,
or of the entire deployment.

As an example, there is a sample config template which collects the
hostname and mac addresses for all nodes in the deployment then writes
the data to all Controller nodes.  Something similar to this may be
required to enable creation of the nexus_config in
https://review.openstack.org/#/c/198754/

There's also another, simpler, example which shows how you could share
the output of an OS::Heat::RandomString between nodes.

Change-Id: I8342a238f50142d8c7426f2b96f4ef1635775509