use jsonschemas to validate API requests

Manila could make use of the jsonschema library to validate API requests. Schemas would be defined within the project for each resource dictating the format, attributes, and allowed values for each request entity. This will allow for intelligent HTTP 400 responses instead of ambigious errors. Currently many bad requests to Manila actually return 500s which may result in a user believing something is wrong with Manila instead of realizing a bad request.

There is at least some prior examples of this in OpenStack, currently glance does this to a small extent.