Add cargo make ami-public and ami-private targets

[zmrow]

Description of changes:

dc4d53b5 Add `cargo make ami-public` and `ami-private` targets
1d806774 pubsys: Move region_from_string to common aws module
e0988d67 Allow JSON output of registered AMIs
293d5b24 rustfmt pubsys client module

Testing done:

• Made multiple AMIs public and private
  • @tjkirch launched an AMI made public from @zmrow 's account
  • Checked permissions on all AMIs before and after cargo make ami-public/private
• Verified that a missing AMI will cause an error

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[tjkirch]

^ This push rebases to fix the conflict from #1034, and rewords the commit messages per @bcressey.

[tjkirch]

^ Addresses @bcressey's comment above; per offline discussion, rather than an exact match, we check that the requested regions are a subset of the ones in the JSON file, in case a user only wants to publish in some of their regions at first.

[webern]

Nice!

[tjkirch]

^ This push addresses @webern's comment about the inaccurate description of the regions parameter. I think the other comments are OK.