render severity configurations (#1512) (#1511)

[beckjake]

Fixes #1511 and #1512 by rendering the "severity" field in test configurations.

[drewbanin]

I found one random issue when poking around with this. If you use a severity setting like this:

version: 2

models:
  - name: my_model
    columns:
      - name: id
        tests:
          - not_null:
              severity: "{{ 'WARN' if var('prod', false) }}"

then dbt will fail with:

'NoneType' object has no attribute 'get'

via

    return render_template(template, ctx, node)
  File "/Users/drew/fishtown/dbt/core/dbt/clients/jinja.py", line 334, in render_template
    return template.render(ctx)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/asyncsupport.py", line 76, in render
    return original_render(self, *args, **kwargs)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/_compat.py", line 37, in reraise
    raise value.with_traceback(tb)
  File "dbt-d3e2f3354c22a85d92b076ce", line 1, in top-level template code
    from __future__ import division, generator_stop
  File "/Users/drew/fishtown/dbt/core/dbt/clients/jinja.py", line 266, in __str__
    self._fail_or_warn_with_undefined_error()
  File "/Users/drew/fishtown/dbt/core/dbt/clients/jinja.py", line 244, in _fail_or_warn_with_undefined_error
    self.node.get('resource_type'),
AttributeError: 'NoneType' object has no attribute 'get'

I think that the 'WARN' if var('prod', false) raises a warning, and something in _fail_or_warn_with_undefined_error doesn't play nice with tests here. I can make a new issue for this, but wanted to point it out here

[drewbanin]

One comment about a relatively specific edge-case. Otherwise, LGTM!