Added the service_ensure parameter

The service_ensure parameter is  passed down to the Service subclass. Defaults to running, as expected, but it allows administrators the ability to toggle the service off from puppet. 

Really, all service wrapping modules should include something like this, but for dhcp it's especially important, as it's disruptive to the network to have two servers running with the same configuration, which complicates host migration. 
That's only in puppet 2.6 which (thankfully) no longer needs to be supported.