per-host ssh settings, hostvars in connections

Changes the runner to pass the inject dictionary down into the connection.  The
upshot of this is that it is now possible to use host-level variables at
connection time.  Building on this functionality, it is now possible to specify
hostvars ssh_user and ssh_pass to specify per-host password.

SECURITY NOTE: Credentials in the inventory are not necessarily a good idea.
	       However, since the inventory is already not usually exposed,
	       this is a calculated risk.  When in doubt, just use SSH
               hostkeys.