Feature request: Read CronJob secrets from different namespace than the cluster's

[mreiche]

The CronJobEnvironmentSecretVariables are being read from the cluster namespace by default: https://github.com/zalando/postgres-operator/blob/master/pkg/cluster/k8sres.go#L1205

Should it be possible to define a namespace override for this secret, to prevent secret duplication if multiple clusters to multiple namespace haven been deployed?

Proposal:

type LogicalBackup struct {
	LogicalBackupCronjobEnvironmentSecret     string `name:"logical_backup_cronjob_environment_secret" default:""`
	LogicalBackupCronjobEnvironmentSecretNamespace     string `name:"logical_backup_cronjob_environment_secret_namespace" default:""`
}

func (c *Cluster) getCronjobEnvironmentSecretVariables() ([]v1.EnvVar, error) {
// [..]
	var secretNamespace string

	if c.OpConfig.LogicalBackupCronjobEnvironmentSecretNamespace == "" {
		secretNamespace = c.OpConfig.LogicalBackupCronjobEnvironmentSecretNamespace
	} else {
		secretNamespace = c.Namespace
	}
	
	secret, err := c.KubeClient.Secrets(secretNamespace).Get(