Spec Lite: Switch to Turn Off Schema Validation

problem: While an API is in EXPERIMENTAL status, the schemas may not accurately reflect the content of requests and responses, causing the client to error out.
solution: Introduce a new option controlled by an environment variable that will turn off schema validation.
impacts: None
timeline: Q-1 milestone (week of 16 October 2017)
link: https://blueprints.launchpad.net/python-glanceclient/+spec/no-schema-validation
reviewers: all core reviewers