selftest: move to declaratively specifying environments and their dependencies

selftest: move to declaratively specifying environments and their dependencies

This removes the tangle of code for starting up dependencies, and allows
selftest.pl to query dependencies (hence it can know when things can be shut
down early and how to order environments for optimal memory usage - that patch
not yet submitted).

It also removes the slightly hacky special-casing of the ad_members, and sets
$target->{vars} centrally (so each setup_ function does not need to).

Signed-off-by: Jamie McClymont <jamiemcclymont@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>