alembic: Bad down revision in add_default_from_user script

The down revision wasn't set correct in the add_default_from_user script.
This patch points it to the correct revision.

Change-Id: Ied45786db265a1d4fb350ef0dd33b4d043c9a74d

contrib/ast-db-manage/config/versions/154177371065_add_default_from_user.py

@@ -8,7 +8,7 @@ Create Date: 2015-09-04 14:13:59.195013
 
 # revision identifiers, used by Alembic.
 revision = '154177371065'
-down_revision = '26f10cadc157'
+down_revision = '5a6ccc758633'
 
 from alembic import op
 import sqlalchemy as sa