LU-12597 tests: remove comma_list for MDTs part 1

LU-12597 tests: remove comma_list for MDTs part 1

Most places that use $(mdts_nodes) also call $(comma_list ...) at the
same time.  Rather than returning a space-separated list of nodes from
those functions, instead return a comma-separated list suitable for
use by do_nodes() and similar functions calling pdsh internally.

This avoids the repeated calls to comma_list() and makes the tests
easier to read.  Also, since comma_list() internally deduplicates the
node list, it isn't necessary to do that in the callers.  A few tests
need to convert comma-separated osts_nodes into space-separated nodes.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I31509872350987564ca560ad93fdaba0443ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57183
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>