Add ability to set storeconfig_backends

[ngrundy]

Overview

This pull request adds the ability to set the storeconfigs_backend option. This allows for the selection of alternate backends for storing catalog data.

The change allows the octocatalog-diff command to now accept the paramater --storeconfigs-backend [puppetdb|yaml|json] which sets the terminus for storeconfigs on the underlying puppet catalog compile command.

The default for storeconfigs_backend has been defaulted to puppetdb to maintain backwards compatibility.

Checklist

• Make sure that all of the tests pass, and fix any that don't. Just run rake in your checkout directory, or review the CI job triggered whenever you push to a pull request.
• Make sure that there is 100% test coverage by running rake coverage:spec or ignoring untestable sections of code with # :nocov comments. If you need help getting to 100% coverage please ask; however, don't just submit code with no tests.
• If you have added a new command line option, we would greatly appreciate a corresponding integration test that exercises it from start to finish. This is optional but recommended.
• If you have added any new gem dependencies, make sure those gems are licensed under the MIT or Apache 2.0 license. We cannot add any dependencies on gems licensed under GPL.
• If you have added any new gem dependencies, make sure you've checked in a copy of the .gem file into the vendor/cache directory.

/cc [related issues] [teams and individuals, making sure to mention why you're CC-ing them]

[cjmartian]

LGTM, just had a question - not blocking. Thanks for doing this!