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 theforeman/puppet-dhcp@1ad54b3 and theforeman/puppet-dhcp@c961257.

[alexjfisher]

@domcleal Apologies, but I didn't notice this issue until after I'd merged #96

Does #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.