Validation of input parameters through JSON Schema

Nova, Cinder and Keystone have used JSON Schema to validate input parameters in start of operation itself instead of going ahead and reported error after error occurs.

We should follow same in Manila.

Nova JSON Schema Validation：:
## Validation:

    def validate(self, *args, **kwargs):
        try:
            self.validator.validate(*args, **kwargs)
        except jsonschema.ValidationError as ex:
            # NOTE: For whole OpenStack message consistency, this error
            # message has been written as the similar format of WSME.
            if len(ex.path) > 0:
                detail = _("Invalid input for field/attribute %(path)s."
                           " Value: %(value)s. %(message)s") % {
                               'path': ex.path.pop(), 'value': ex.instance,
                               'message': ex.message
                           }
            else:
                detail = ex.message
            raise exception.ValidationError(detail=detail)
        except TypeError as ex:
            # NOTE: If passing non string value to patternProperties parameter,
            # TypeError happens. Here is for catching the TypeError.
            detail = six.text_type(ex)
            raise exception.ValidationError(detail=detail)

## At API layer:
https://github.com/openstack/nova/blob/master/nova/api/openstack/compute/create_backup.py

    @validation.schema(create_backup.create_backup, '2.1')
    def _create_backup(self, req, id, body):
        """Backup a server instance.

https://github.com/openstack/nova/tree/master/nova/api/openstack/compute/schemas

Cinder JSON Schema Validation：
[1] https://blueprints.launchpad.net/cinder/+spec/json-schema-validation
[2] https://review.openstack.org/#/c/507386/