Wire in hieradata overrides via ExtraConfig for NovaCompute

Wire in hieradata overrides via ExtraConfig for NovaCompute

Adds support for global (ExtraConfig) and role-specific
(NovaComputeExtraConfig) hiera overrides, similar to those added
for the controller.

For example, you can pass an environment file like:
parameters:
  NovaComputeExtraConfig:
    nova::scheduler::filter::ram_allocation_ratio: 1.8
    compute_classes:
    - ::nova::scheduler::filter

This passes a hiera value for ram_allocation_ratio and enables
a class via the include added in https://review.openstack.org/#/c/197908/

Note this also requires https://review.openstack.org/#/c/188772/
or 40-hiera-datafiles incorrectly quotes the list and the
compute_classes part won't work.

Change-Id: Ic33eed1b5e9c33c0d2f6075c65c8d9649b82c8b4