Add ipset element and hashsize tunables

[OpenStack Infra]

https://review.openstack.org/170328
commit b5b919a7a3569ccb93c3d7d523c1edfaeddb7cb9
Author: Brian Haley <email address hidden>
Date: Thu Apr 2 21:11:06 2015 -0400

    Add ipset element and hashsize tunables

    Recently, these messages have been noticed in both tempest
    logs, as well as reported by downstream users syslog:

      Set IPv4915d358d-2c5b-43b5-9862 is full, maxelem 65536 reached

    So the default of 64K is not sufficient enough.

    This change adds two config options to control both the number
    of elements as well as the hashsize, since they should be
    tuned together for best performance. Slightly different
    formats were required for 'ipset create' and 'ipset restore'.

    The default values for these are now set to 131072 (maxelem) and
    2048 (hashsize), which is an increase over their typical default values
    of 65536/1024 (respectively), in order to fix the errors seen in
    the tempest tests.

    DocImpact

    Change-Id: Ic0b5b38a840e737dc6be938230f4052974c8620f
    Closes-bug: #1439817

[Gauvain Pocentek (gpocentek)]

The settings have not been picked up by our tools and are not listed in the config-ref.

[Gauvain Pocentek (gpocentek)]

Actually these settings are not in the Kilo branch, so they should be imported for liberty.