Fix hosts file entries - remove duplicates/stale entries

Adds to tasks in the updatehostsfile play that will do the following to the
/etc/hosts file:
* Removes any entries that match the IP of current host, but with a different
  hostname
* Removes any entries that match the hostname of the current host but with a
  different IP

This will prevent duplicate entries and other /etc/hosts file related
strangeness.

Fixes #13
Merge pull request #17 from andymcc/hosts_fix

Fix hosts file entries - remove duplicates/stale entries