IMPALA-624: Inserts should respect changes in partition location

Impala would ignore changes in a partition's location (by ALTER TABLE
... SET LOCATION ...).

Change-Id: I9fdc1f09f9d848aa1a4ade3d4f35f8de9cbd18a5
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1647
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins