ignore_null option for mysql external pillar

```
The ignore_null option can be used to change the overwrite behavior so that
only non-NULL values in subsequent results will overwrite.  This can be used
to selectively overwrite default values.

.. code-block:: yaml

  ext_pillar:
    - mysql:
        - query: "SELECT pillar,value FROM pillars WHERE minion_id = 'default' and minion_id != %s"
          depth: 2
        - query: "SELECT pillar,value FROM pillars WHERE minion_id = %s"
          depth: 2
          ignore_null: True
```