schema: add keyfile configuration for ssh disks

schema: add keyfile configuration for ssh disks

Authenticating via key file to an ssh server is often preferable to
logging in via password. In order to support this functionality add a
new <identity> xml element for ssh disks that allows the user to specify
a keyfile and username. Example configuration:

    <disk type='network'>
      <source protocol='ssh' ...>
        <identity keyfile='/path/to/id_rsa' username='myusername'/>
        ...
      </source>
    ...
    </disk>

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