Add config_defaults hash parameter

[michaeltchapman]

Allow sites to specify a baseline set of config items to be applied to
all nodes via config_defaults that can be overridden via hash
merge with config_hash. This doesn't change the current behavior
for users, but allows users running both agents and servers
to consolidate defaults for both roles in a single key instead
of repeating the defaults merely to change a small number of config
options such as server: true and bootstrap_expect on server nodes.

[michaeltchapman]

I have an example here if this seems a bit strange:

https://github.com/michaeltchapman/vagrant-consul/blob/master/hiera/data/modules/consul.yaml
https://github.com/michaeltchapman/vagrant-consul/blob/master/hiera/data/roles/server.yaml

So I'm not having to repeat the same common data across roles.

[solarkennedy]

I would normally insist that this kind of code+logic live in what I would call "profile_consul", but being a hash, I understand why this would be useful. I believe this PR does make this module more usable in this regard.