LU-13620 tests: pool_add_targets() fix

LU-13620 tests: pool_add_targets() fix

Fix pool_add_targets to don't fail if number of
OSTs is >= 10 - lctl expects them in a hex view.
Check result of "lctl pool_add". Pass only if
the result is either 0 or 17(EEXIST).
Finally, make pool_add_targets to check that were
added only requested OSTs, i.e. don't fail to add
OST0 if OST1 is already in a pool.

Test-Parameters: trivial testlist=ost-pools
HPE-bug-id: LUS-8723
HPE-bug-id: LUS-8941
Change-Id: I841b3db3a89dbc86075cd23b7d71764ffb849181
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-on: https://review.whamcloud.com/38803
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>