rgw/sfs: update user credentials on env variable change

[jecluis]

In a k8s context, it is expected that if a given service secret is updated, then it will be reflected in said service's application. This is, apparently, the behavior of other services like databases and what not.

We would need to ensure that, if a given environment variable is changed (say, S3GW_USER_ACCESS_KEY for instance), we reflect that in the store's database.