Add db table for Glance Metadata

Add db table for Glance Metadata

This commit implements the blueprint
https://blueprints.launchpad.net/cinder/+spec/retain-glance-metadata-for-billing

It creates the new table volume_glance_metadata in the cinder
database, provides the CRUD methods for it, and populates the table
when a volume or snapshot is created from a Glance image.

Patch set 2: remove superflous line

Patch set 3: Fix incorrect column types in sqlalchemy/models.py

Patch set 4: Define exception class GlanceMetadataExists

Change-Id: I8f98f6eaae005a33bfd49cea783774407b7aa120