guests/{ubuntu,debian}: fix change_host_name for trailing dots [GH-2610]

When `/etc/hosts` contained a FQDN with a trailing dot, the `\b` in the
sed expression would not match, since dot is not considered to be a word
character.

For a fix, we're simply removing the `\b` constraint so we are more
lenient about matching the `/etc/hosts` line we'd like to replace.