LU-12597 tests: allow comma-separated node lists

LU-12597 tests: allow comma-separated node lists

Allow some functions that deal with space-separated node lists to
also accept comma-separated node lists, to prepare for a future
where $(osts_nodes) and $(mdts_nodes) will return comma-separated
lists already, instead of having to call comma_list each time.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I36e5a2a0814fd6564ca560ad93fdaba0423ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54340
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>