Specify the MetadefNamespace.namespace column is not nullable

The metadef_namespaces table definition indicates the namespace
column as not accepting nulls. The related MetadefNamespace ORM class
should also indicate that the namespace column does not accept nulls
with "nullable=False" in the column definition.

Change-Id: I681747b6579a2284f23bf154889a61c639b0616d
Closes-Bug: 1367619
(cherry picked from commit 10e858d57be5e04615f390b3060c8aeadaeed954)
Merge "Specify the MetadefNamespace.namespace column is not nullable" into proposed/juno