Initialise the "metadata" table in the db.

def _init_metadata_table():
    """Initialise the "metadata" table in the db."""
    _metadata_table = sqlalchemy.Table(
        'metadata', _METADATA,
        sqlalchemy.Column(
            'job_id', sqlalchemy.ForeignKey("jobs.job_id", ondelete="CASCADE"),
            nullable=False, primary_key=True),
        sqlalchemy.Column('key', sqlalchemy.UnicodeText, primary_key=True),
        sqlalchemy.Column('value', sqlalchemy.UnicodeText, index=True),
        sqlalchemy.Column('type', sqlalchemy.UnicodeText),
        )
    return _metadata_table