Add PostgreSQL support to test migrations.

This commit adds support for running the migrations tests
with a PostgreSQL backend.

Migration 012 was changed because PostgreSQL's constraint
drop uses a different name to identify the foreign key
to be removed. The migration now handles it properly for
both postgres and mysql.

Change-Id: Ie8850608fc5f4bccda57a0b12c3b4adc8d1f61d0
Add pre and check phases to test migration 004

Assure that the new column is included and with the right size and no data is
missed during the process.

Change-Id: I836e9ab5ec8219f2056d755585a9e58facd23901
Add pre and check phases to test migration 005

Migration 005 changes size collumn from integer to BigInteger. This checks if
everything goes fine in the migration.

Change-Id: Ia4f12e3d43d7fcc31dc999bc1ad5dcd0610df597
Merge "Add PostgreSQL support to test migrations."
Merge "Add pre and check phases to test migration 004"
Merge "Add pre and check phases to test migration 005"
Merge remote-tracking branch 'm_src_repo/webob-issue' into build-temp