Modify the displayed format for gnt-os diagnose

Get rid of the tabulated form and create a simplier indented structure.
Now a valid OS looks something like:

OS: foo [global status: partial valid]
  Node: gsdtest4.euw.corp.google.com, status: valid (path: /usr/local/lib/ganeti/os/foo)
  Node: gsdtest5.euw.corp.google.com, status: valid (path: /usr/local/lib/ganeti/os/foo)
    [hidden] path: /usr/local/lib/ganeti/os2/foo, status: 'ganeti_api_version' file not found (ENOENT)
  Node: gsdtest3.euw.corp.google.com, status: 'ganeti_api_version' file not found (ENOENT) (path: /usr/local/lib/ganeti/os/foo)

Reviewed-By: imsnah