Allow ignoring successful commands in "gnt-cluster command"

In some cases it is useful to ignore the output of and avoid mentioning
successful commands. One would be when looking for a certain string in
a file:

  $ gnt-cluster command egrep -q '^testing$' /etc/...

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>