Machine-identifiable placeholder values

[mattsb42-aws]

From #12:

Minor quibble about using "REPLACEME": this is an arbitrary value that will be swapped out as Pipeformer runs and puts in the real user inputs, so couldn't it be more specific to 1) the data being populated (e.g. "SECRETS_MANAGER_SECRET_PLACEHOLDER") and/or 2) structured to be more clearly computer-readable placeholder input (e.g. "@@PLACEHOLDER@@")?

This could let you do a sort of type checking/validation that the input value is being swapped for the placeholder token expected and add some error checking bounds around token substitution.

We need to investigate this a bit more before committing to a path, but I think this is a good idea.

This will provide a mechanism to allow pipeformer to determine where in a deployment we have reached, as well as providing an option for determining whether we need to re-prompt users for input values on pipeline updates.

[mattsb42-aws]

Closing down project: #52