Operator build should allow custom namespace

Currently the Operator installs into the `default` namespace. The Operator can be installed into a custom namespace, but only by modifying all instances of the `namespace` field in the [Operator manifest](https://github.com/cockroachdb/cockroach-operator/blob/master/manifests/operator.yaml) and having the user manually create that namespace on the K8s cluster. Instead, the Operator should automatically create a specified custom namespace.

cc @keith-mcclellan

Relates to #493