Evaluate use of defaultdict for resources section handling in config.

[em-pe]

What about using from collections import defaultdict with
resources: Optional[Dict[str, ResourcesConfig]] = defaultdict(lambda: ResourcesConfig(cpu="500m", memory="1024Mi"))

it would also simplify (or even make not needed) the resources_for function.

Originally posted by @marrrcin in #11 (comment)

[Lasica]

It could simplify it a bit but it currently does some more work. It think it makes sense to rethink a bit resources configuration to enable changing all of the available params (including docker images if one wishes to) #129

We may need to have a fresh look at this as we have grouping nodes as a feature to consider as well.