Foreman » Smart Proxy

If someone specifies settings in YAML like this:

```yaml
tls_disabled_versions:
- 1.1
```

Ruby will interpret it as a float. Currently, it must be quoted.
@Ivan Necas [added a to_s to the dynflow smart proxy plugin](https://github.com/theforeman/smart_proxy_dynflow/pull/43#pullrequestreview-92019342) to fix this,
this commit makes smart proxy behavior the same.