LU-15672 tests: fix typo in facet_failover_host()

LU-15672 tests: fix typo in facet_failover_host()

mgsfailover_HOST is to be equal to mds1failover_HOST if mds and
mgs are combined and mds1failover_HOST is not empty.

Fixes: 36475ba5fc ("LU-9272 tests: fix for facet_failover mgs")
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-10746
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: Iadd27e977d95794a53149a99380296329469452c
Reviewed-on: https://review.whamcloud.com/46892
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>