## Bug description

This is what a default feature flag looks like:
<img width="577" alt="empty feature flag" src="https://user-images.githubusercontent.com/4550621/113333154-63f36a00-9322-11eb-8fb0-71b0cd865181.png">
It has no rollout percentage specified, nor filters, so it says "Rolling out to 100% of users". However I've experienced that in fact in this situation the flag is rolled out to *nobody*.

CC @paolodamico 

## Expected behavior

Flag should work the way it says.

## How to reproduce

1. Create a feature flag and leave it as is, supposedly completely rolled out.
2. Try to use it.
3. Discover that the flag is not rolled out.

## Environment

- [x] PostHog Cloud
- [x] self-hosted PostHog, version/commit: master (8f4ae2b)

#### *Thank you* for your bug report – we love squashing them!

posthog/test/test_feature_flag.py
feature_flag = self.create_feature_flag(filters={"groups": [{"rollout_percentage": 50}]})
        self.assertTrue(feature_flag.distinct_id_matches("example_id"))
        self.assertFalse(feature_flag.distinct_id_matches("another_id"))
    def test_empty_group(self):
        feature_flag = self.create_feature_flag(filters={"groups": [{}]})
        self.assertFalse(feature_flag.distinct_id_matches("example_id"))
        self.assertFalse(feature_flag.distinct_id_matches("another_id"))
    def test_complicated_flag(self):
        Person.objects.create(
            team=self.team, distinct_ids=["test_id"], properties={"email": "test@posthog.com"},
        )
        feature_flag = self.create_feature_flag(
            filters={
                "groups": [
                    {
                        "properties": [
                            {"key": "email", "type": "person", "value": "test@posthog.com", "operator": "exact"}
                        ],
                        "rollout_percentage": 100,
                    },
                    {"rollout_percentage": 50},
                ]
            }
        )
        self.assertTrue(feature_flag.distinct_id_matches("test_id"))
        self.assertTrue(feature_flag.distinct_id_matches("example_id"))
        self.assertFalse(feature_flag.distinct_id_matches("another_id"))
    def test_multi_property_filters(self):