Support a hotfix process

[cgwalters]

Suppose that an OS vendor wants to provide a custom tree just to a specific user that contains a hotfix.

Of course one mechanism for hot fixes could be #107
but let's presume for this bug that we are focusing on the "new tree" approach. (Although possibly provided as a delta, knowing the customer is running the latest tree).

Regardless, let's assume that this tree data is provided out of band - in a tarball, Docker container, or whatever that is only accessible to that user.

We want a user experience here where the user can:

• Switch to the tree
• The tree is clearly labeled as a hotfix
• There is an easy command to swtich back to the mainline tree (e.g. rpm-ostree upgrade should clearly show a message that you're on a dead-end hotfix stream)

Things are obviously simpler here if the OS vendor provides online-accessible hotfixes, possibly in the same repository; then one can use rebase to switch to it.

[cgwalters]

Calling this one fixed by ostreedev/ostree#212