Nailgun Release columns should be 'NOT NULLABLE'

[Ihor Kalnytskyi]

In Nailgun there's a releases table, which has a bunch of columns that might be 'NOT NULLABLE' [1]. Some of them are:

* networks_metadata
* attributes_metadata
* volumes_metadata
* modes_metadata
* roles_metadata
* wizard_metadata
* deployment_tasks
* vmware_attributes_metadata

There're a lot of code in Nailgun which expects it to be a proper JSON, however some tests create release entries with 'NULL' in that field, which might break some code. It causes a random test failures, depending on which order tests are executing, since our tests do not clear temporary created records.

In order to prevent such mistakes, it'd be better to add 'NOT NULLABLE' constraint for that field, and force test authors to properly fill that field.

[1] http://xsnippet.org/361430/raw/