Allow modify_etc_hosts to be changed

Allow modify_etc_hosts to be changed

The modify_etc_hosts options, enabling the cluster to modify the /etc/hosts
files of nodes, and to keep them in sync, could only be set at cluster init
time.

With this commit it can now be changed through modify_etc_hosts as well.

Fixes Issue 206.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>