LU-327 cleanup the client import of mgc

LU-327 cleanup the client import of mgc

the client import will not have been cleaned if it is setup but never
connected, adding such cleanup code in mgc_precleanup.

define a new inline function "obd_cleanup_client_import" in "obd_class.h"
to be used by mgc, mdc and osc.

Change-Id: If033de3fe81b9a565a7af40a7b93e395fb0bd659
Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/917
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>