[config] change relay/para builders to be more explicit and less confusing.

[pepoviola]

Currently we have with_node (in relay) and with_collator (in para) but the node is by default a validator and can be confusing. To reduce the friction we should:

In relay builder:
add:
with_validator (make authority, pass --validator flag)
change:
add_node (full node)

In para builder:
add:
add_node (full node)
change:
with_collator (make authority, pass --collator flag)

And remove the methods that allow to set the role of the node.

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cc: @bkchr