(QE-519) beaker should raise error if host does not have a platform set

- beaker tries to do crazy things when hosts aren't on supported
  platforms, let's check that option right at the start
documentation for supported HOST settings
enforcing platform format of OS-VERSION-ARCH

- added spec tests to ensure that we correctly sort out bad platform
  names
- will need to write an associated patch to update our node
  configuration files to meet the new standard
Merge pull request #72 from anodelman/check-platform-set

(QE-519) beaker should raise error if host does not have a platform set