Simplify k5test.py environments

Simplify k5test.py environments

The initial k5test.py design, copied from the dejagnu suite, is to
create config files and environments for four expected roles: client,
server, master, and slave.  This approach exaggerates the complexity
of the common case, where the configurations don't need to vary, and
limits us to having just one slave for kprop/iprop tests.

Instead, create just one configuration by default, and add a
special_env() method which sets up a differently configured
environment for the few test cases which need one.  The run_as_*()
methods are collapsed into just run(), which accepts an optional
argument for the environment returned by special_env().