Add primary key constraint to alembic_version

Add primary key constraint to alembic_version

The alembic_version table, when initially created, now establishes a
primary key constraint on the "version_num" column, to suit database
engines that don't support tables without primary keys.   This behavior
can be controlled using the parameter
:paramref:`.EnvironmentContext.configure.version_table_pk`.  Note that
this change only applies to the initial creation of the alembic_version
table; it does not impact any existing alembic_version table already
present.

Change-Id: Ic947f0f97373b2e6695e06c9b2ad6c8a9789fcb2
Fixes: #406