check for pre-genesis validator in join-network cmd

[tzemanovic]

Depends on anoma/anoma#1053

The join-network cmd can be used with --genesis-validator {alias}, but if omitted, it will setup non-validator node. This can lead to some confusion when someone intends to setup a genesis validator node, but forgets to use this option.

When --genesis-validator is not specified, the join-network cmd could check for pre-genesis existence and warn the user with e.g.

"Pre-genesis validator detected, but requested to join network as a non-validator node. Did you perhaps forget to specify the --genesis-validator argument?"

[tzemanovic]

done in #2088 - we print "No validator keys are being used. Make sure you didn't forget to specify --genesis-validator?"