Improve error messages in cluster verify/OS

A few issues in the clarity of the error messages are fixed:

- "ERROR: node node3: OS API version lenny-image": no preposition
  between the parameter type and the OS name, changed to "for
  lenny-image"

- "API version lenny-image differs from reference node node1: 10, 5
  vs. 10, 20, 5, 15": parameters not sorted in display

- "OS variants list lenny-image differs from reference node node1:
  vs. default, i386": empty sets are not clearly delimited, changed to
  add [] around the sets: "node node1: [] vs. [default, i386]"

- "OS parameters lenny-image differs from reference node node1:
  vs. (u'dhcp', u'Whether to enable (yes) or disable (dhcp)')": ugly
  formatting in the OS parameters list, as we used to just "%s" the
  tuple; now it is "reference node node1: [] vs. [dhcp: Whether to
  enable (yes) or disable (dhcp)]"

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>