Default public schema

[jerith]

All Postgres databases are created with a default public schema that Postgres implicitly uses whenever a schema isn't specified. However, the operator currently operates only on schemas that are explicitly listed in Postgres CRs, which means that PostgresUsers with READ of WRITE privilege levels don't have access to the public schema by default. (This isn't a problem for the OWNER privilege level, because database ownership is sufficient for anything that schema ownership allows.)

Given that non-schema-aware applications are extremely common, I think it would make sense for the schemas list in a Postgres CR to default to ["public"] rather than an empty list. (However, this isn't sufficient on its own. See #61 for details.)

Thanks for all the work on this operator so far. We've found it to be extremely useful. :-)