Metadef schema column name is a reserved word in MySQL

The metadef_properties and metadef_objects tables both have
a column named schema. Unfortunately, schema is a reserved word
in some relational database products, including MySQL and PostgreSQL.
The metadef_properties.schema and metadef_objects.schema
columns should be renamed to a non reserved word.

Conflicts:
	glance/db/sqlalchemy/metadata.py
	glance/tests/unit/test_migrations.py

Change-Id: I9c1b497d2b09b9282a83bd8c19c32edfa4dd159f
Closes-Bug: 1378968
Merge "Metadef schema column name is a reserved word in MySQL" into proposed/juno