LU-12514 utils: add "lustre_tgt" filesystem type

LU-12514 utils: add "lustre_tgt" filesystem type

Add a "lustre_tgt" filesystem type separate from the "lustre"
filesystem type for forward compatibility to allow the client and
server mount code to be disentangled.  This does not actually make
any changes to the mount code itself, but allows testing interop
and fallback after an upgrade if the new filesystem type is used.

Remove the check for the "lustre_lite" filesystem type from
llapi_is_lustre_mnttype() since that was unused since Lustre 1.4,
and replace it with a check for filesystem type "lustre_tgt".

Test-Parameters: trivial testlist=conf-sanity
Change-Id: If635834dfff502e6ecf8a85a2665de09efd52372
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36165
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: Maloo <maloo@whamcloud.com>