Update glancev2 to use parent-col-name for tags table

Previously the tags schema would look like this:

$ openstack congress datasource table schema show  glancev2 tags
+-----------+-------------+
| name      | description |
+-----------+-------------+
| parent_id | None        |
| tag       | None        |
+-----------+-------------+

$ openstack  congress datasource table schema show glancev2 tags
+----------+-------------+
| name     | description |
+----------+-------------+
| image_id | None        |
| tag      | None        |
+----------+-------------+

This makes it obviously that, that column is an image_id.

Change-Id: Idc48f323a927afcf56ef6e573b1b4ae8bac108b3
Merge "Update glancev2 to use parent-col-name for tags table"