Right now, the sss_ssh_knownhostsproxy client pieces will update the users' ~/.ssh/sss_known_hosts file. But it makes more sense that we should have the SSH Responder update the /etc/ssh/sss_known_hosts file globally for the system instead.

This will accomplish several things:
1) It eliminates the risk of concurrent clients editing the hosts file because the Responder is serialized
2) It will reduce LDAP lookups in general, since multiple users will not need to look up the same hosts
3) It will reduce the scope of the client libraries to be more consistent with our other sss_client features.