LU-16672 tests: auster node.yml labels Alma and Rocky as CentOS

LU-16672 tests: auster node.yml labels Alma and Rocky as CentOS

release() assumes a node with /etc/centos-release is CentOS. This patch
removes that assumption and uses the name in the centos-release file.
Corrected the os-release code to strip off the last word if present.

Lustre-change: https://review.whamcloud.com/50442
Lustre-commit: TBD (from 3d6c37836b4bb7b1e6ea90bb7aacf4715e44c667)

Test-Parameters: trivial
Other-Id: Ia5acbce3351ca23f4d9265d1aaf8d952a2c8b502
Change-Id: Id16ec38d3530c4ece4fbdbb56c23af24d4c55b99
Signed-off-by: Charlie Olmstead <charlie@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50617
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>