Replace instances of "getRequiredProperty" with accessors to respective classes

For example, there is the property getter:

```
environment.getRequiredProperty("mesos.resources.cpus", Double.class)
```

It should use the getter:
MesosConfigProperties.getResources.getCpus()

This is so that other accessors also don't have to use the static getRequiredProperty method.

(Another eg. scale isn't a field in ResourcesConfigProperties)