LU-15471 tests: use propper facet device

LU-15471 tests: use propper facet device

Tests which stop facet are to recalculate facet device
after stop as it changes when device mapper is used:
the device-mapper device is removed and facet device is
restored:
  stop () ->
     elif dm_flakey_supported $facet; then
        if [[ -n ${!failover_host} && ${!failover_host} != ${!host} ]]
           dm_cleanup_dev $facet ->
              unexport_dm_dev $facet

Without this fix sanity 17m, 17n 804 tests fail on failover
setup with:
  Cannot resolve path /dev/mapper/mds1_flakey
  e2fsck: No such file or directory while trying
                     to open /dev/mapper/mds1_flakey
and sanity 228b, 256, tests fail because of:
  mount: /dev/mapper/mds1_flakey: failed to setup loop device:
                     No such file or directory
  losetup: /dev/mapper/mds1_flakey: failed to set up loop device

To reproduce the failures -- just run:
  ONLY="17m 17n 228b 256 804" sh sanity.sh
on failover setup where mds1_HOST != mds1failover_HOST.

Fixes: 54b9e3f789 ("LU-684 tests: replace dev_read_only patch with dm-flakey")
Test-Parameters: trivial testlist=sanity
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-9808
Change-Id: I02ce9d7cb7cf804fe0596d9aad7f995242c4b3af
Reviewed-on: https://review.whamcloud.com/46254
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Vladimir Saveliev <vlaidimir.saveliev@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>