Replace entry in /etc/hosts instead of appending

[c0deaddict]

When using kitchen-qemu in combination with cloudinit, a duplicate entry in the /etc/hosts file is created by the script. For example, this is the contents of /etc/hosts after a converge:

# Your system has configured 'manage_etc_hosts' as True.
# As a result, if you wish for changes to this file to persist
# then you will need to either
# a.) make changes to the master file in /etc/cloud/templates/hosts.debian.tmpl
# b.) change or remove the value of 'manage_etc_hosts' in
#     /etc/cloud/cloud.cfg or cloud-config from user-data
#
127.0.1.1 kitchen-ci.test kitchen-ci
127.0.0.1 localhost

# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts

127.0.0.1 new-hostname.local new-hostname

The problem is that the hostname command gives back localhost, and not new-hostname.

This PR replaces the existing hostname entry with the new hostname. I'm not 100% if this works for all Linux distros. I've tested this for Debian 8, 9 and 10.