Target environments in Spin templates

[itowlson]

When we have target environment definitions rolled out, we should reference them in Spin templates.

• Should the choice be baked into the template, or can the user choose?
  • If baked in, we risk it not being what the user wants or expects
  • If choose, we risk demonstrating our superior intellect by asking a question the user cannot answer (particularly new users)
• Should it be possible to opt out?
  • This would probably require a small tweak to the template system (because its text substitution system is too naive)

We'll need to consider how downstream vendors can use the feature. We don't want to privilege vendor environments in neutral templates. A possible approach is for vendors to fork templates with references to their proprietary environments (and possibly front those with plugins helpers to install and/or instantiate the vendor templates), but that's hand waving right now!