Feature Request: --dryrun support to inspect parameter values post-resolver

[JoeArmstrong]

Subject of the issue

I would like to be able to inspect the values of parameters to be used for both update and create prior to invoking the sceptre actions. The support of a '--dryrun' option to sceptre is a way to allow sceptre to run, performing all actions other than cloudformation deploy or update operation - dumping the parameters to be used (after all resolvers have been run) so that they can be inspected.

The use of the change-set ability works for already deployed stacks but results in an error for stacks that have not yet been deployed resulting in the inability to inspect parameter values prior to stack creation.

[jfalkenstein]

@JoeArmstrong , I wonder to what extent the diff command would address the needs that you desire --dry-run for. Have you tried using the diff command that was released in 2.7.0?