Kubernetes Client - Config - Some properties should be optional

[gsmet]

Nearly all the properties are required, whereas I'm pretty sure they should be optional (typically the proxy stuff).

Please use Optional where it makes sense.

@geoand @gastaldi could you have a look? It's not high priority but I just included the generated doc and it ends up with required everywhere.

I'm also wondering if most of the config should be runtime instead of build time?

[geoand]

I'll check it out