Provide default for fail2ban whitelist

Allows this class to be used in Vagrant -- without `env.production.yaml` in
the deployment repo. Otherwise the following happens:
```
Error: Could not find data item ci_environment::fail2ban::whitelist_ips in any Hiera data file and no default supplied at /tmp/vagrant-puppet/modules-0/ci_environment/manifests/fail
2ban.pp:6 on node transition-logs-1
```

Also changes the explicit `hiera()` call to a class param, which is more
standard and easier to use.
Merge pull request #99 from alphagov/fail2ban_whitelist_defaults

LGTM :+1: