Fix check_003 method.

This commit fixes the check on migration 003 in test_migrations.
Previously, when this method was added it didn't check that the
data in the type column previously in the images table was moved
correctly into the image_properties table. This commit adds these
data checks.

Change-Id: Ia3906c8497218c8a3d2a13b0c99a6aa566977ed4
Fix circular import in glance/db/sqlalchemy

Fix running test_migration solely.

api.py depends on models.py that depends on api.py.
Import statements in these files are such that api.py
must be imported before models.py. That's why usually
test_migrations doesn't fail when all tests are launched.
However, thus test_migration depends on test running order
making testing procedure unreliable, including parallel testing.

Move glance.db.sqlalchemy.api import to the only place
where it's required in models.py

Change-Id: Ib20dde8edd3473595aac1ca8bbaa5a0d3116a697
Fixes: bug #1154058
Merge "Fix circular import in glance/db/sqlalchemy"
Merge "Fix check_003 method."