Invoker limits should not be hard-coded.

When limiting actions in terms of nproc, nfile and cgroup memory usage we currently hard-code those limits.

They are defined in

```
ContainerUtils.scala
MemoryLimit.scala
```

I propose we do not hard-code those limits. They should be adjustable via config.

My suggestion:
1. Put them in `ansible/group_vars/all`
2. Write them to whisk.properties
3. Have Invoker read them from consul.

We should also think about making the action time limit configurable as well.