include advanced upgrade documentation for Katello P3 to P4
In https://github.com/Katello/katello.org/pull/294#discussion_r77226103, Eric mentioned a few more advanced upgrade scenarios to document. They include:
- how to spin up Puppet 4 capsule, use it for testing of clients, then upgrade original capsule
- how to spin up Puppet 4 capsule, migrate hosts "puppet only" part to it, upgrade original Capsule and migrate hosts back to it (we could include things like heres how to do this with REX or here is some Ansible to help with this)
- how to spin up Puppet 4 capsule and migrate everything from original Capsule to it
NOTE: this issue can be a considered a general "document advanced upgrade scenarios" issue. It does not necessarily have to hit all the above bullet points exactly as stated.