LU-18286 tests: auster node.yml labels rocky9 as RHEL

LU-18286 tests: auster node.yml labels rocky9 as RHEL

release() assumes a node with /etc/redhat-release is RHEL.
This patch removes reading this file in favor of os-release.
Reading of centos-release (if present) still required as
os-release for centos distros doesn't include minor version.

Test-Parameters: trivial
Signed-off-by: Charlie Olmstead <charlie@whamcloud.com>
Change-Id: I888f8eeacaf843120b2beb134292047b3907a9a6
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56580
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Alex Deiter <adeiter@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>