restapi: Add collection for operating systems

This patch adds a new top level "operatingsystems" read only collection:

  GET /operatingsystems
  <operating_systems>
    <operating_system id="0">
      <name>other</name>
      <description>Other OS</description>
    </operating_system>
    ...
  </operating_systems>

  GET /operatingsystems/0
  <operating_system id="0">
    <name>other</name>
    <description>Other OS</description>
  </operating_system>

Change-Id: Ibae9977a6fbfc8728ee66e705f0594f230ff47ba
Bug-Url: https://bugzilla.redhat.com/1050243
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>