LU-639 obdclass: wait obd cleanup before mount

LU-639 obdclass: wait obd cleanup before mount

Obd device cleanup is executed by obd zombie thread, and umount thread
can return successfully before obd zomebie finishes its job. In some
cases, especially in test cases, a test may starts before last tests
finishes obd cleanup, this patch makes mount thread wait for obd
zombie finishes its job.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I881be5de18960867c36e8c4e4180c0c594d88a01
Reviewed-on: http://review.whamcloud.com/1896
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>