Makes tasks owner not nullable in models.py

The tasks table scheme in migration for adding tasks to Glance DB has a
flag for disabling from setting a NULL owner.

In order to keep the behavior consistent across the glance codebase,
this patch adds owner as not nullable in the models.py.

Fixes bug: 1257949

Change-Id: Id9fff4277f427b7d83d80b5a0a8029968f70e0df
Merge "Makes tasks owner not nullable in models.py"