LU-911 ost: ost_handle() to refill environment on every requests

LU-911 ost: ost_handle() to refill environment on every requests

OST threads hold environment (lu_env) for request handling. when
a last instance of some device (say ofd or osd) gone, all keys
registered by the device are released, but enviroments are kept.
then at device instantiation the environments aren't repopulated.
this patch fixes this.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ib691f22955d40335535fcf3013d62badb24f7f49
Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1850
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>