LU-13275 tests: enhance racer with failover

LU-13275 tests: enhance racer with failover

Patch adds the possibilty to run facet_failovier <random_facet>
if RACER_FAILOVER is set to true.
The type of facets are set over FAIL_TARGETS variable,
default value is "MDS OST".

Test-Parameters: trivial \
envdefinitions=RACER_FAILOVER=true testlist=racer
Cray-bug-id: LUS-8280
Signed-off-by: Elena Gryaznova <c17455@cray.com>
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@hpe.com>
Change-Id: Iacb6ac855bf77d7c6930fdbb7ec192ecc46264b3
Reviewed-on: https://review.whamcloud.com/37652
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>