fuzzit: make the submit phase a bit more robust

fuzzit: make the submit phase a bit more robust

The submit phase of the Fuzzit Travis job has been spuriously failing
for some time with various (and usually pretty hidden) errors, like:

```
./fuzzit create job --type regression ...
2020/04/23 17:02:12 please set env variable FUZZIT_API_KEY or pass --api-key. API Key for you account: ...
```

```
./fuzzit create job --type regression ...
2020/04/23 11:36:53 Creating job...
2020/04/23 11:36:54 Uploading fuzzer...
2020/04/23 11:36:54 Job created successfully
2020/04/23 11:36:54 Get https://...&action=create: read tcp x.x.x.x:39674->x.x.x.x:443: read: connection reset by peer
```

```
./fuzzit create job --type regression ...
2020/04/22 18:09:15 Creating job...
2020/04/22 18:09:16 Uploading fuzzer...
2020/04/22 18:09:37 Job created successfully
2020/04/22 18:09:37 500 Internal Server Error
```

etc.

Let's retry each submit job up to three times to (hopefully) mitigate this.