TLS disabled versions in smart proxy should be converted to string
If someone specifies settings in YAML like this:
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.