Throw clear error when metrics_options missing

[rpappalax]

LambdaFunctionFailed
{"error":"KeyError","cause":"{\"stackTrace\": [[\"/var/task/handler.py\", 18, \"ensure_metrics_available\", \"return runner.ensure_metrics_available()\"], [\"/var/task/ardere/step_functions.py\", 239, \"ensure_metrics_available\", \"self.ecs.create_metrics_service(self.event[\\\"metrics_options\\\"])\"], [\"/var/task/ardere/aws.py\", 359, \"create_metrics_service\", \"\\\"\\\"\\\" % (self.grafana_admin_user, self.grafana_admin_password) # noqa\"], [\"/var/task/ardere/aws.py\", 194, \"grafana_admin_user\", \"return self._plan[\\\"metrics_options\\\"][\\\"dashboard\\\"][\\\"admin_user\\\"]\"]], \"errorType\": \"KeyError\", \"errorMessage\": \"'dashboard'\"}"} 

[pjenvey]

We enable metrics when no options are specified, however the defaults don't include any dashboard entry, hence a KeyError here