LU-12597 tests: clean up comma_list in test-framework

LU-12597 tests: clean up comma_list in test-framework

Start cleaning up callers of comma_list() in test-framework where it
is no longer needed since osts_nodes() and tgts_nodes() already
return a comma-separated list of nodes.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Icae1342f69af4393cc1ece8f92318c6fa03ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56636
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>