Run tests on PostgreSQL and MySQL too

In addition to running tests with different Python and SQLAlchemy
versions, they should also be run on different DB backends, which
are used in production (PostgreSQL and MySQL).

This patch:
  - modifies test_db.cfg to run tests on PostgreSQL and MySQL
    (Jenkins Slave credentials are used here, to ensure these
    tests are always run by Jenkins gate); if a backend is not
    available, test cases will be skipped for it
  - concurrency is set to 1 (sharing of the one MySQL or PostgreSQL
    DB among different test runner processes would lead to
    race conditions)
  - fixes tests dropping FK columns in MySQL: in earlier MySQL
    versions dropping a column that is a part of a FK constraint
    would lead to dropping of the FK too. As of MySQL 5.5 that's
    not the case anymore: if one tries to drop such column, he/she
    will get a very obscure error (something like "Error on rename
    of './openstack_citest/#sql-4af_aa2' to './openstack_citest/tmp_adddropcol'
    (errno: 150)") '\nALTER TABLE tmp_adddropcol DROP COLUMN r2').
    So the solution if to drop FK constraints first, and only then
    the columns it is made up of

Change-Id: I8c5d2874c83e7df46da69969ed54d85437c849e7