Add `ssl3` switch to load balancer create/update

SSLv3 is deprecated due to security vulnerabilities. Load balancers no
longer support it by default.

Passing `--ssl3` during creation or an update can enable it.

Existing load balancers have it enabled for backward compatibility but
it can be disabled with `--no-ssl3`
Rename SSLv3 arg to `sslv3`

Fits in better with the API parameter we use.
Merge pull request #64 from brightbox/sslv3_option

Add `ssl3` switch to load balancer create/update