Common pre-upgrade steps

On all of your hosts:

If you are running Puppet as configured by Staypuft, you must disable it:
```
 # systemctl stop puppet
 # systemctl disable puppet
```

This ensures that the Staypuft-configured puppet will not revert changes made as part of the upgrade process.

Install the Juno yum repository.
Upgrade the openstack-selinux package, if available:
```
 yum upgrade openstack-selinux
```
This is necessary to ensure that the upgraded services will run correctly on a system with SELinux enabled.