advertise ".local" sites to host OS via mDNS

Uses [avahi][1] to advertise [mDNS][2] (a.k.a. Bonjure, Zeroconf) 
aliases for each defined site that has a name matching the pattern 
`*.local`.  Each site is advertised as a CNAME record that points back
to the primary hostname of the VM.  This allows each site to be 
resolved by name from the host OS, without modification the host 
machine's `/etc/hosts` file.  Also, since the CNAMEs point directly to 
the VM itself, sites can be accessed via standard ports (eg. 80, 
443, etc), without requiring use of mapped ports on the host machine.
For example: the default site configured in `Homestead.yaml`, can 
now be accessed from a browser on the host machine using the 
following simple URI:

    http://mysite.homestead.local/

The mdns CNAMEs also resolve correctly within the VM, so application
code can reference itself, if necessary, using the same domain name 
that was used to call it from the browser on the host machine.

This code has been tested and works on OSX host machines.  It should 
also work correctly on any host OS that supports mdns/bonjure/zeroconf, 
including Windows.  Linux hosts should be supported as well, if they are
running an mdns resolver (eg. [libnss-mdns][3]).

[1]: http://avahi.org/
[2]: http://www.multicastdns.org/
[3]: http://packages.ubuntu.com/trusty/libnss-mdns