Stringize values given to env_vars, for when we gsub them on output

I found that when I did:
```ruby
  env_vars UNICORN_COUNT: 5
```
I got an exception when [this line](https://github.com/newrelic/centurion/blob/e3916e0f065158189118e2d4cfc9530661cf824e/lib/centurion/deploy.rb#L107) maps '%DOCKER_HOSTNAME%' assuming that the value will be a string.

So, this PR stringizes values when env_vars gets them. (The test also ensures that keys will be stringized, which they already were).