Set MONGO_RAISE_ON_ERROR=False if creating MongoDB indexes
manually and Alerta API should not fail to start if automatic
index creation fails.

* Add cloudwatch webhook test

* Use built-in request.json instead of json.loads()

Grafana webhook alert properties can be controlled by overriding the
default ones with webhook url parameters:

Example (split on multiple lines for readability):

  https://alerta/webhooks/grafana?api-key=xxx
      &environment=Production
      &event_type=performanceAlert
      &group=Performance
      &origin=Grafana
      &service=Grafana
      &severity=major

* Fix auto-unshelve for postgres

* Fix auto-unshelve for mongo

(#528)

* Allow admin to set customer value

* Add tests for customer assignment