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 commit 0.7.2-16-gc670d1d the _index_identifier() implementation
was copied from sqlalchemy, as that function was renamed
in sqlalchemy 0.8.  Instead handle call the renamed function
when appropriate, to decouple ourselves from the sqlalchemy
implementation.

Change-Id: I97b22c20d96758fc5b6bd55318218edb26c5b5d0

(fixes issue 125)