Allow Drydock blueprints to define and use custom fields

Summary: This allows Drydock blueprints to define custom fields for blueprint settings.

Test Plan: Pulled out of EC2 allocator diff.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10224