This patch adds the initial support for DB2 10.5 to migrate. It
includes:

1. The dialect implementation for DB2.
2. The dialect registration with the visitor.
3. Code to parse the engine name in version.py.
4. A new dependency on ibm_db_sa in test-requirements.txt.
5. A connection string in test_db.cfg for ibm_db_sa.

Part of blueprint add-db2-support

Co-authored-by: Sheng Bo Hou <sbhou@cn.ibm.com>
Co-authored-by: Thuy Christenson <thuy@us.ibm.com>
Co-authored-by: Rahul Priyadarshi <rahul.priyadarshi@in.ibm.com>
Change-Id: I745ec615487b1b06c5d1a09ea316f376d66ee4c0

Version 0.8 of SQLAlchemy added support of indexes
on expressions in addition to plain table columns,
which changed the way indexes are created.

This broke support of dropping columns of composite
indexes for SQLite: due to limitations of ALTER in
SQLite every time a column is dropped, we recreate
the whole table without the given column; if a
column is a part of a composite index, we change the
index definition to omit that column and then indexes
are recreated too.

SQLAlchemy versions starting from 0.8 no more pay
attention to 'columns' attribute of Index instances
when generating DDL for indexes, so when one of columns
of a composite index is dropped, we try to create a
new index on the column that doesn't exist anymore,
which of course fails.

Closes-Bug: #1241038

Change-Id: I777b8ce36e36f49bfb0889908811a063cf1a527b

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

Change-Id: Ib541ac9d6b397300e34ca8b65aad459b612858c3

fixes #72

excludeTables correctly

Fixes issue 140