LU-4218 tests: fix for facet_host()

LU-4218 tests: fix for facet_host()

When ost_HOST is not set (ost1_HOST, ost2_HOST, etc are used),
facet_host() maps ostNfailover facet (when ostNfailover_HOST is not
set) to default value of ost_HOST, etc hostname.
With this change, facet_host() maps ostNfailover facet to value of
ostN_HOST if it is set, otherwise it resorts to $ost_HOST.

Xyratex-bug-id: MRP-1118
Signed-off-by: Vladimir Saveliev <vladimir_saveliev@xyratex.com>
Change-Id: I23dd7cb0d7de02740ab60f9f86442bc92a4a0300
Reviewed-on: http://review.whamcloud.com/8194
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>