`dnssec-keygen` uses `/dev/random` by default, but this is slow in some
scenarios where `/dev/urandom` is both faster and provides sufficient utility.
Allow override via the `bind::random_device` hiera key.
This adds a 'dnssec' parameter to the bind::zone define which causes the module
to generate keys and sign the zone. Some caveats and breaking changes:
1) Existing non-signed zones will have to be manually moved and signed
2) Signed zones are treated as dynamic
