(PUP-2610) Maintain context during application exit

Because of the way rack servers get configured by puppet, the
application system needs to maintain any context information that is
setup while starting even after the application has exited. This is
because the rack app is created by calling the "master" application and
using the return value (which threads all the way back through the
application and command line layers) as the rack app. When the contexts
were removed on the way out of the application they also removed the
setup for how environments were loaded.

This also required that there be a way of undoing a series of context
pushes without knowing how many had been done. For this there is now a
mark and rollback capability in the Puppet::Context system. This is used
in the tests to always return to the context that was present at the
start of every test.