Include Metadata Defs Concepts in Dev Docs

The http://docs.openstack.org/developer/glance/
site currently doesn't include the Juno Metadata
Definitions concepts.  This patch adds
an overview of the concepts to this site.

This provides a synopis of the concepts in:
https://github.com/openstack/glance-specs/blob/master/specs/juno/metadata-schema-catalog.rst

DocImpact
Closes-Bug: 1367432
Related-Bug: 1367908
Related-Bug: 1363615
Related-Bug: 1366286
Related-Bug: 1363383
Change-Id: Iad4d388cbbf2f63fa243d04d35032de0cb0bc1b4
Refactor test_migrations module

Refactored migration tests to use OpportunisticTestCase, removed
unused code and ``test_migrations.conf`` file.

The main feature of this approach is to create a new database with
random name for each migration test.  This will avoid migration tests of
race conditions and reduce tests intersection. After this change, database
``openstack_citest`` will be used only for initial connection to the database.

``test_migrations.conf`` file not required anymore, because we create test
database for migration test, so we no longer need to keep database credentials.

Partial-Bug: #1368274

Change-Id: Ib1d6dfae62cc60f814c01d07adc53f68e7c234f6
Run tests with default concurrency 0

Setting concurrency to 0 would enable Glance tests to be run using the
the total number of cores on the system rather than by a single one.

This would, by default, speed up the run time of tests for developers
who are not aware about it. Also, any jobs which run these tests for
Continuous Integration would be benefitted without having to deal with
the machine level logic.

Closes-Bug: #1368274

Change-Id: I0842b0c0be1272d479fad9ef7609cd8a29231cb5
Merge "Include Metadata Defs Concepts in Dev Docs"
Merge "Refactor test_migrations module"
Merge "Run tests with default concurrency 0"