Explicit foreign key indexes.

Overview:
MySQL requires indexes on foreign keys. Such an index is created on the
referencing table automatically if it does not exist.

Motivation:
MySQL names foreign keys indexes differently dependent from itself
version. Implicit fk indexes is MySQL specific behavior, so it should be
ignored during comparision. We plan to use Alembic to compare models and
migrations I0758c7e09d1d46ce870c3f94f76c2a177955e143. Alembic doesn't support
5.5 naming convention. So we need consistent index name for all MySQL versions
to be easily ignored.

Solution:
A new migration script renames incorrectly named fk indexes on MySQL.

Reproduction:
Stop execution when foregn keys will be initialized. See schema with
your MySQL shell client. With 5.5, indexes which was not named
explicitly, will have a name such as <tablename>_<columnname>_fk, with 5.1 it
will be <columnname>.

Change-Id: I01ba0c8856afbda59394395ac319bf83a55ee2be
Partial-Bug: #1292591
Merge "Explicit foreign key indexes."