[installer] Make all config values with sensible defaults optional

[corneliusludmann]

In my opinion, we should make all config values that have sensible default values optional in the config file. E.g., it should be totally fine to use a config like this:

apiVersion: v1
domain: gitpod.my-domain.example

That would also make generating config values for e.g. replicated easier.

see also #8266 (comment)

[Pothulapati]

@corneliusludmann Totally agree with the suggestion! I was confused initially too, and why I need to pass all the defaults even when its the init command that is providing me those values.

We should make those optional, and override them if they are passed by the user! I'll start working on this!