chmod ssh known hosts file in set_ssh_known_host (salt.modules.ssh)

#12437 implemented management of gobal ssh known hosts file (e.g. /etc/ssh/ssh_known_hosts). Unfortunately this file isn't world-readable yet.

This change always sets 0644 for known hosts regardless whether there are global or user-specific since those files contain public data. I also haven't seen an user-specific known_hosts file which is not world-readable so far anyway.

See also http://www.openbsd.org/cgi-bin/man.cgi?query=ssh&sektion=1 (File: /etc/ssh/ssh_known_hosts)