ctdb-scripts: Add caching function for public IPs

ctdb-scripts: Add caching function for public IPs

This is way more complicated than I would like but, as per the
comment, this is due to complexities in the way public IPs work.  The
main consumer will be statd-callout, which will then be able to run as
a non-root user.

Also generate the cache file in test code, whenever the PNN is set.
However, this can cause "ctdb ip" to generate a fake IP layout before
public IPs are setup.  So, have the "ctdb ip" stub generate the IP
layout every time it is run to avoid it being stale.

Signed-off-by: Martin Schwenke <mschwenke@ddn.com>
Reviewed-by: Volker Lendecke <vl@samba.org>