fix(mount) make self.instances more robust
fix(mount) make self.instances more robust

Different versions of mount(.s3ql) could report `user` instead of
`user_id`. We tune our regex, and use defaults of `0`.

Please note that even if extract a user, or group name, the type's
`munge` blocks will normalise them down to numbers, for consistency.

Brainsware/puppet-s3ql