LU-18020 tests: define CONFIG for do_rpc_nodes

LU-18020 tests: define CONFIG for do_rpc_nodes

This allows correct test environment definitions when CONFIG is
defined but not NAME. This is useful when configuration files live
some place other than lustre/tests/cfg

Test-Parameters: trivial
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: If1be2123d5fc60a1bdf210627bc5d0b9bdbf3daa
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55695
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Elena <elena.gryaznova@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>