pubsys: add dry-run option to SSM commands

[sam-berning]

Request

Add a flag to pubsys ssm and pubsys promote-ssm that causes those commands to output the SSM parameters that would be pushed, but not actually push them.

Why it's useful

This will allow users to ensure that the parameters they are pushing look correct before pushing them. It also decouples generating the files used to validate the parameters from actually publishing the parameters.