Update azure ohai plugin to use SharedConfig.xml on linux for node attributes.

When we grab content of sharedConfig (https://github.com/rightscale/right_link/pull/459/files#diff-e3463eb110494b0084b1751aead185fcR113) we explicitly provide container and instance id. That is why I think, we should not have 2 instances here. Also QA proved that provided information is correct.