Add flag to disable hostname lookup

In testing / buildinging DCOS we've found that we need to set --hostname explicitly on the masters. For our uses IP and `hostname` must always be the same thing.

More in general, under certain circumstances, dynamic lookup of hostname, while successful, provides undesirable results; we would also like, in those circumstances, be able to just set the hostname to the chosen
IP address (possibly set via the -- ip_discovery_command method).

We suggest adding a --no-hostname-lookup.
Note that we can introduce this flag as --hostname-lookup with a default to 'true' (which is the current semantics) and that way someone can do --no-hostname-lookup or --hostname-lookup=false.