[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_141016_refuse_to_run_on_incompatible_hosts'

Please review the following changes:
  * 2998fa4 Refactor OsInspector
  * e4593b9 System#cat_file -> #read_file, make it os agnostic on local systems
  * 55b2831 Abort Machinery when we're running on an incompatible host
  * cc03fad Add helper method which ensures that we're running on a compatible host
  * 40d46db Make specific os detection methods private, introduce "#get_os" instead
  * 5f0f427 Mention supported operating system in README