Add 'hostname', 'domain' and 'fqdn' matchers.

Add three matchers to the Host resource type.

its(:hostname) can match the short name
its(:domain) matches the domain portion
its(:fqdn) matches the fully qualified name

Example
```
describe host('localhost') do
  its(:hostname) { should eq 'server' }
  its(:domain) { should eq 'example.com' }
  its(:fqdn) { should eq 'server.example.com' }
end
```