drop constraint before modifying the path (#2312)

Based on [this discussion](https://chat.openshift.io/developers/pl/6m4cojtiotdoueqot8uww5ybpe) we first remove the unique name index on the `iterations` and `areas` table before we modify the data. 

This should fix the migration issue that was visible here: openshiftio/saas-openshiftio#1082

migration/sql-files/105-update-root-area-and-iteration-path-field.sql

@@ -2,6 +2,10 @@
 UPDATE iterations SET path=text2ltree(concat(path, concat('.',replace(cast(id as text), '-', '_')))) WHERE path!='' AND path IS NOT NULL;
 UPDATE areas SET path=text2ltree(concat(path, concat('.',replace(cast(id as text), '-', '_')))) WHERE path!='' AND path IS NOT NULL;
 
+-- drop constraints
+ALTER TABLE iterations DROP CONSTRAINT iterations_name_space_id_path_unique;
+ALTER TABLE areas DROP CONSTRAINT areas_name_space_id_path_unique;
+
 -- update root iteration and area paths to use converted ids
 UPDATE iterations SET path=text2ltree(replace(cast(id as text), '-', '_')) WHERE path='' OR PATH IS NULL;
 UPDATE areas SET path=text2ltree(replace(cast(id as text), '-', '_')) WHERE path='' OR PATH IS NULL;
@@ -10,10 +14,6 @@ UPDATE areas SET path=text2ltree(replace(cast(id as text), '-', '_')) WHERE path
 ALTER TABLE iterations ALTER COLUMN path SET NOT NULL;
 ALTER TABLE areas ALTER COLUMN path SET NOT NULL;
 
--- drop constraints
-ALTER TABLE iterations DROP CONSTRAINT iterations_name_space_id_path_unique;
-ALTER TABLE areas DROP CONSTRAINT areas_name_space_id_path_unique;
-
 ALTER TABLE iterations ADD CONSTRAINT non_empty_path_check CHECK (trim(path::text) <> '');
 ALTER TABLE areas ADD CONSTRAINT non_empty_path_check CHECK (trim(path::text) <> '');