Update main.yml (#7267)

````
TASK [bootstrap-os : Enable RHEL 8 repos] ***************************************************************************************************************************************************************************************************
fatal: [node6]: FAILED! => {"changed": false, "msg": "This system has no repositories available through subscriptions"}
fatal: [node7]: FAILED! => {"changed": false, "msg": "This system has no repositories available through subscriptions"}
fatal: [node1]: FAILED! => {"changed": false, "msg": "This system has no repositories available through subscriptions"}


root@node1:/kubespray# cat /etc/os-release
NAME="Ubuntu"
VERSION="18.04.5 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.5 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic
root@node1:/kubespray#
```

roles/bootstrap-os/tasks/main.yml

@@ -10,7 +10,7 @@
   when: '"CentOS" in os_release.stdout or "Oracle" in os_release.stdout'
 
 - include_tasks: bootstrap-redhat.yml
-  when: '"Red Hat Enterprise Linux" in os_release.stdout'
+  when: '"Red Hat Enterprise Linux" in os_release.stdout and "CentOS" not in os_release.stdout'
 
 - include_tasks: bootstrap-clearlinux.yml
   when: '"Clear Linux OS" in os_release.stdout'