LU-15260 tests: numfailovers() fix

LU-15260 tests: numfailovers() fix

Patch fixes numfailovers() to use comma
separated MDTS list correctly. Without this fix
in newer bash version we see the following error:
  line 69: mds1,mds2,mds3,mds4_nums: bad substitution

Fixes: a7a2133bfa ("b=18696 new RECOVERY_RANDOM_SCALE test")
Fixes: b594948509 ("TT-59 remove . and - from the node name")
Test-Parameters: trivial testlist=recovery-random-scale
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-10619
Change-Id: I4c28e3c62cada60dc1241948dc4e969e0e10ce9a
Reviewed-on: https://review.whamcloud.com/45633
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>