Refactor userdata template to account for different bootstrap requirements

[ahaysx]

The userdata template to bootstrap an EC2 VM is currently pretty static. I ran into an issue where the image I was using required changes to the template before it would start correctly. My worry is that this change could break starting other images.

If you're allowing the end-user to configure custom images then you might consider allowing them to configure custom bootstrapping that is specific to their images. Otherwise, the userdata template has to support "all" images in a heavy-handed way or only supports "supported" images which is limiting.

Not sure if this is related to #45 which seems to cover just augmenting the base userdata script.

[yadudoc]

Hey @ahayschi, as you pointed out this gets pretty difficult once you drop the assumption that the instance is ubuntu-based. So far we've limited ourselves to the simplest/common cases which is why the instance setup is pretty rigid. If you have a use-case in mind, that could help design this provider to be more flexible.

[ahaysx]

Hey Yadu thanks for the reply. I'm just experimenting and don't have an immediate use case in mind. It just occurred to me when I had to update the template to work with the public ubuntu AMI that I wad forcing everyone else to install these additional packages that they might not need and/or might break their build. I definitely understand wanting to keep it simple and support a blessed set of AMIs. If this is the case, maybe I can contribute some clarification on this in the docs?