schema: add configuration for host verification of ssh disks

schema: add configuration for host verification of ssh disks

In order to make ssh disks usable, we need to be able to validate a
remote host. To do this, add a <knownHosts> xml element for ssh disks to
allow the user to specify a location for a file that contains known host
keys. Implementation to follow.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>