Metadef Property and Object schema columns should use JSONEncodedDict

The MetadefProperty and MetadefObject ORM classes currently specify the
JSON schema columns as type Text. It is preferred to use the
JSONEncodedDict Type Decorator instead. This fix also includes necessary
code changes to remove JSON encoding/decoding that was previously done
in other layers. Fixes for unit tests involving the schema columns are
also included.

Closes-Bug: 1368479

Conflicts:
	glance/db/__init__.py
	glance/db/sqlalchemy/models_metadef.py

Change-Id: I2c574210f8d62c77a438afab83ff80f3e5bd2fe7
(cherry picked from commit 824d9620b0b90483baf45981d2cb328855943e06)
Merge "Metadef Property and Object schema columns should use JSONEncodedDict" into proposed/juno