LU-6867 test: detect active facet based on current state

LU-6867 test: detect active facet based on current state

Lustre failover tests can not be ran test-by-test
on the setup with ${facet}_HOST != ${facet}failover_HOST
because of t-f does not restore facet state.
t-f keeps this info in "${facet}active" files, which are created
when facet_failover() is executed first time in the test session.
Before facet_failover() executed these files are empty and
active facet is ${facet} by default.
In case when tests are executed test-by-test the active facet is
${facet}failover after 1st test completed, and 2nd test is started
having ${facet}failover active without this info stored in
${facet}active files.

Patch contains the following changes:
- add the active facet detection based on current lustre state;
- fix sanity-hsm defect: exist with error if agt${n}1_HOST is empty.

Signed-off-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Xyratex-bug-id: MRP-2680
Reviewed-by: Alexander Lezhoev <alexander.lezhoev@seagate.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@seagate.com>
Change-Id: Ie42baaa55a6433596e6004d16eb5c18ae2ef7479
Reviewed-on: https://review.whamcloud.com/15638
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alexander Lezhoev <c17454@cray.com>
Reviewed-by: Andriy Skulysh <c17819@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>