Add search domains option to dhcp::pool

[domcleal]

dhcp::pool should have a search domains parameter that sets the domain-search option in the pool to a list of DNS domains. It should accept a single domain or an array.

Based on https://github.com/theforeman/puppet-dhcp/commit/1ad54b36f7dbb2876aa84ec28d63401de4c7e863 and https://github.com/theforeman/puppet-dhcp/commit/c9612576d49c277cbc9ae58f4bc0776b287aecd6.

[alexjfisher]

@domcleal Apologies, but I didn't notice this issue until after I'd merged https://github.com/voxpupuli/puppet-dhcp/pull/96

Does https://github.com/voxpupuli/puppet-dhcp/pull/96 work for you, or do you need improvements? I guess since we've not done a release yet, we can change the parameter name if needed.

[domcleal]

Ideally there would (also) be a parameter on dhcp::pool to match the impl that we merged into theforeman. I don't think we can reasonably match parameter names between the two modules, so I think it's fine - we can do the same to dhcp::pool.