ovs-lib: Return the exit status of ovs-ctl in ovs_ctl().

ovs-lib: Return the exit status of ovs-ctl in ovs_ctl().

commit 46528f78e5c(debian, rhel, xenserver: Ability to collect ovs-ctl logs)
made changes in the startup scripts such that the o/p of ovs-ctl is logged
into ovs-ctl.log. But it had an unintended consequence that the exit status
of ovs-ctl was no longer returned. We would always return success(the exit
status of tee).

With this commit, we return the exit status of ovs-ctl instead of tee.
Code referenced from: (line wrapped).
http://unix.stackexchange.com/questions/14270/\
get-exit-status-of-process-thats-piped-to-another/70675#70675)

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Signed-off-by: Duffie Cooley <dcooley@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>