LU-12597 tests: return comma-separated osts_nodes()

LU-12597 tests: return comma-separated osts_nodes()

Start to return comma-separated OSTs list from osts_nodes(),
to avoid the redundant calling of comma_list() for each user.

Add the tgts_nodes() helper to print combined MDT and OST nodes to
avoid duplicate nodes vs. separate mdts_nodes() + osts_nodes().

Fix the few places that do not call comma_list() on osts_nodes()
output afterward, and code style in sanity-sec test_31.

Fix a minor bug in the error handling of sanity check_stats() where
it was calling "osts-nodes" instead of "osts_nodes".

Later patches will clean up all of the callers of osts_nodes,
but there are too many places it is used to do in a single patch.

Fixes: f0324c5c2f ("LU-14992 tests: sanity/replay-vbr mkdir on MDT0")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I7ae3278321e7552dc2afd5fbb9f48033af3ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56635
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>