Add ipset element and hashsize tunables

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