Support running playgrounds in content with different options

This is similar to #17 but instead of describing a solution, it describes requirements.

I, as a content author, need a way to run playgrounds with different options in contents.

One actual example: I wrote a couple tutorials about OpenBao/Vault. Instead of publishing them as separate tutorials, I used tabbed content, since they are mostly the same. Currently users have to enter a value in an input choice to start the chosen service.

However, this is a bit inconvenient, because:

• users get the prompt before choosing a service
• both services are actually installed which may be confusing
• since services are installed in init tasks, it takes a "lot" of time to install both

An ideal solution would be being able to choose which service to run before starting the playground.

A couple ideas:

Expose playground init conditions to content (as described in #17)

Since custom rootfs support is coming to the platform, it might be better to somehow choose between two rootfs (so installation can be done upfront instead of init tasks).

@iximiuz suggested choosing between playgrounds instead of rootfs.

One potential problem with that if we still want init conditions support, it might complicate the implementation.