Add an interface for querying OS versions and flavors

[vkarak]

Similar to what we introduced for Cray's CLE in #1416, but more general, in order to cover other OSes.

[teojgo]

For additional information regarding the os version, kernel etc, the platform python module can be used.

[vkarak]

I don't think that this is actually needed especially now that we add support for specifying features and extras in partitions and environments.