Duplicate entries of an environment variable should make cluster in error state

[SaaldjorMike]

Currently it's possible to specify the same environment variable multiple times in the HumioCluster resource, but that implicitly means the first one takes precedence over any following entries for the same environment variable.

This means the cluster sometimes ends up with a different unexpected behaviour as you might not have noticed the environment variable was already present in the list when adding another entry of the same environment variable.

It would be better to disallow the user to specify the same environment variable multiple times and instead communicate some ConfigError state to the user which clear logs about which environment variable(s) have been specified multiple times.