LU-10815 tests: all_mdts_nodes() all_osts_nodes() defect

When mds_HOST and ost_HOST are not set (mds1_HOST,
ost1_HOST, ost2_HOST, etc are used), all_mdts_nodes() and
all_osts_nodes() have client in the list set due to cfg/local.sh:
  mds_HOST=${mds_HOST:-$(hostname)}
  ost_HOST=${ost_HOST:-$(hostname)}
With this change these functions include mds_HOST and ost_HOST
into the list only if mdsN_HOST and ostN_HOST are not set.

Signed-off-by: Elena Gryaznova <c17455@cray.com>
Cray-bug-id: LUS-5784
Reviewed-by: Alexander Boyko <c17825@cray.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Change-Id: I1ccfc94a186db98fbf87698cb8fc7d08ee008fdc
Reviewed-on: https://review.whamcloud.com/33342
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>