Verify iprop test results more thoroughly

In k5test.py, allow run_kadminl to take an environment argument.  In
t_iprop.py, perform some queries on the slaves after each propagation
to spot-check that it got modifications from master.  Use a helper
function to check serial numbers for conciseness.
Propagate policy changes over iprop via full dump

Since iprop cannot carry policy changes, force a full resync to happen
each time a policy change occurs.  Based on a patch from
Richard Basch <basch@alum.mit.edu>.

ticket: 7522