perftest: add a new medley test

perftest: add a new medley test

This is something of a rewrite of ad_dc_performance.py with more
search tests and a rebalancing of others. For example, the users are
added in three lots of 2000 using varying methods rather than 5 of
1000 using ldap, reducing duplication thus clarifying the results.

Links are added in more realistic patterns with groups of varying
size.

To save time, the database is not cleaned up. Usually perftests are
run with TESTS= restriction to a single suite, but in case this is not
done, this suite is run last.

The ad_dc_performance suite is not replaced so that comparisons with
old test sequences are still possible.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>