(Maint) Don't change permissions on /etc/hosts

Previously, the acceptance test was backing up /etc/hosts, modifying it,
verifying that puppet resource host returned all of the host records,
and then moving the backup over /etc/hosts. In doing so, the permissions
on /etc/hosts were overwritten.

Since puppet runs as root on agent nodes, the test was setting the mode
of /etc/hosts to 0600 (based on root's umask).

However, when the node is also a master, the puppet master cannot read
the file, since it runs as puppet. This causes problems for any later
test that uses with_master_running_on, e.g.
ticket_7117_broke_env_criteria_authconf.rb:

Could not resolve 192.168.100.114: Permission denied - /etc/hosts

This commit changes the test so that the existing /etc/hosts file is
truncated and the old contents restored, thereby preserving its old
mode.

Ideally, it would be nice to not have to overwrite /etc/hosts at all,
e.g. puppet resource host --param target=/my/tmp/file, but this only
works for a single host resource, not when querying all of them.