Automatically set postgrescluster.enabled to false if postgresql.type != 'postgrescluster'

[batpad]

Currently, if you set postgresql.type to external-plaintext for example in a values over-ride, you will get an error as it still tries to create the PostgresCluster resource.

Therefore, one also needs to set postgrescluster.enabled to false to work with an externally managed database.

This feels a bit redundant. It would be nice if the postgres cluster setup was automatically disabled if postgresql.type was set to anything other than postgrescluster.

Really not a huge deal, just an ergonomics thing - noticed this while setting up eoapi-k8s for WRI.

cc @emmanuelmathot @pantierra