Support CentOS Stream 9

This patch includes changes required to run devstack on CentOS Stream 9
which has been already published in official repos by CentOS team [1]:

- Add RDO deps repository for CS9.
- remove xinetd package from installation for swift. Note that
  rsync-daemon is installed which should work fine.
- Replace genisoimage by xorriso in CS9.
- Use /etc/os-release to identify the distro in CS9 as it doesn't
  provide lsb_release command.
- Use pip from rpm package instead of from get-pip.py as done in Fedora.
- Add non-voting job devstack-platform-centos-9-stream to the check
  pipeline.

Change-Id: Ic67cddabd5069211dc0611994b8b8360bcd61bef
diff --git a/files/rpms/n-cpu b/files/rpms/n-cpu
index 68e5472..7ce5a72 100644
--- a/files/rpms/n-cpu
+++ b/files/rpms/n-cpu
@@ -1,9 +1,10 @@
 cryptsetup
 dosfstools
-genisoimage
+genisoimage # not:rhel9
 iscsi-initiator-utils
 libosinfo
 lvm2
 sg3_utils
 # Stuff for diablo volumes
 sysfsutils
+xorriso # not:rhel8