Convert AllNodesExtraConfig to support composable roles

Convert AllNodesExtraConfig to support composable roles

This adjusts the interface to OS::TripleO::AllNodesExtraConfig so
it supports custom/composable/optional roles.

Note this does break backwards compatibility, and I can't see any way
to avoid that.  I've converted the in-tree templates, and we'll have
to document carefully and or provide a script (or automated conversion
via mistral perhaps?) to allow folks to easily adjust any out of tree
templates to the new format.

Basically you just have to:

1. Remove all the *_servers parameters, replace with one "servers"
   json parameter

2. Replace references to e.g "controller_servers" with "servers, Controller"
   which does a path-based lookup into the json map provided by overcloud.yaml

Change-Id: I5eebf853646b2f6300d6b542fcd4f43e82d3b413
Partially-Implements: blueprint custom-roles