Fix for mysql alembic migration

In case of MySQL database when column is altered the existing type needs to be
specified.

Signed-off-by: Michal Konečný <mkonecny@redhat.com>

alembic/versions/c0bffa4e8fbc_token_if_in_commit_flag_can_be_null.py

@@ -19,6 +19,7 @@ def upgrade():
     op.alter_column(
         'commit_flags',
         column_name='token_id',
+        existing_type=sa.String(64),
         nullable=False,
         existing_nullable=True
     )
@@ -28,6 +29,7 @@ def downgrade():
     op.alter_column(
         'commit_flags',
         column_name='token_id',
+        existing_type=sa.String(64),
         nullable=True,
         existing_nullable=False
     )