no need to edit /etc/hostname

The ansible hostname module edits the necessary files: /etc/hostname on
Debian and /etc/sysconfig/network on RedHat so we don't need to do it
here.
fix a couple of bugs in /etc/hosts editing

We were writing a line for the IP address of each host in the inventory,
but writing the same host and fqdn for each one, so we got entries like:

10.68.90.142 ec2.caboteria.org ec2
192.168.1.79 ec2.caboteria.org ec2
192.168.1.88 ec2.caboteria.org ec2

The regexp didn't match what we were writing to the file so we would
write a new set of lines each time ansible ran.