If the server port is 0 it doesn't mean you don't want *any* container
because there might still be servlet context aware components in the
classpath. Better to try and stop the server from actually starting.

This change is a step in the right direction: it prevents the server
from starting in the start() method (even though for technical reasons
it may have already been started and stopped in initialize()).

N.B. Tomcat at least uses port=0 as a signal it should pick a random
port. Pertty useful feature that we may want to capitalize on at some
point (on the other hand having our own port scanner would be useful
generally too).