Don't import keyring unless needed

[adorton-adobe]

In config.py, the keyring library should be imported only if needed. The import statement should probably be moved to get_credential(), inside the if secure_value_key condition starting on line 601 (see https://github.com/adobe-apiplatform/user-sync.py/blob/v2/user_sync/config.py#L601).

This change will make it easier to run the sync tool on Linux systems. Headless server versions of popular distributions such as Ubuntu or CentOS may not include dbus, which is typically packaged with desktop environments. dbus is used by keyring in Linux to communicate with the credential storage backend.