Adding GITLAB_GRAVATAR_SSL_URL

@xflash I think you would need to use a different delimiter character in the sed commands here. Since the urls would have the '/' characters in them, sed would get confused. You would need to use some other delimiter character like '|', eg.

```bash
sudo -u git -H sed 's|{{GITLAB_GRAVATAR_URL}}|'"${GITLAB_GRAVATAR_URL}"'|' -i config/gitlab.yml
sudo -u git -H sed 's|{{GITLAB_GRAVATAR_SEC_URL}}|'"${GITLAB_GRAVATAR_SEC_URL}"'|' -i config/gitlab.yml
```

I have not tested this yet, but I think that is going to be the case.  Have you tested setting those parameters in your tests?