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