alembic: Fix migration.

The 81b01a191a46_pjsip_add_contact_reg_server.py script was attempting
to use UniqueConstraint and failing. It was not imported and after
importing it also continued to fail.

I've changed the script to use the explicit name of the constraint
instead.

Change-Id: I2438b0be90b7ce583b47dd27983c0c1a02cea5b9

contrib/ast-db-manage/config/versions/81b01a191a46_pjsip_add_contact_reg_server.py

@@ -16,7 +16,7 @@ import sqlalchemy as sa
 
 def upgrade():
     op.add_column('ps_contacts', sa.Column('reg_server', sa.String(20)))
-    op.drop_constraint(UniqueConstraint('id'), 'ps_contacts', type_='unique')
+    op.drop_constraint('id', 'ps_contacts', type_='unique')
     op.create_unique_constraint('ps_contacts_uq', 'ps_contacts', ['id','reg_server'])
 
 def downgrade():