cliutils passing sortby=None breaks older versions of prettytable

Older versions (e.g. 0.6.1) of PrettyTable.get_string fails with:

  "Invalid field name: None!"

when sortby is set to None (to indicate no sorting is required).

The prettytable version requirement is generally set to:

  prettytable>=0.6,<0.8

for example in Oslo and python-ceilometerclient respectively:

  https://github.com/openstack/oslo-incubator/blob/master/requirements.txt#L8
  https://github.com/openstack/python-ceilometerclient/blob/master/requirements.txt#L5

Now, as it happens the global requirements have been very recently updated to increase the lower bound for another reason:

  https://github.com/openstack/requirements/commit/45b937a2

However this change has not percolated out to the client libraries as yet, and in any case would still cause a practical issue as most major distros are still on prettytable 0.6.1.