Fixed fqdn handling on debian/ubuntu

The issue was that the old method simply didn't work.  When the hosts
file should look like:
    127.0.1.1   host.fqdn.com host
It looked like:
    127.0.1.1   host.fqdn.com host old.fqdn.com old
Or this if the user didn't set a fqdn
    127.0.1.1   host host old.fqdn.com old

This patch fixes that.
Allow to change the DHCP_HOSTNAME on RedHat
Merge pull request #2267 from fcharlier/fcharlier/dhcp_hostname_redhat

guests/redhat: Allow to change the DHCP_HOSTNAME on RedHat
Update CHANGELOG
Merge pull request #2254 from Caustic/fix-hostname-debian

guests/debian,ubuntu: Fixed fqdn handling
Update CHANGELOG
website/docs: mention installing virtualbox