Consistently get CWD across apps and arbiter.

The Arbiter is smart about getting the CWD; first it checks the CWD
environment (which doesn't resolve symlinks), then it falls back to the
python os.getcwd() (which does resolve symlinks). However, the Arbiter
is the only place that does this, which will then do the right thing
when we reexec. However, when reloading the Arbiter, it won't pick up
changes if the symlink has changed.

By changing the *app.py entry points to also use the same method for
determining the CWD, we'll insert a symlink path into the first location
in sys.path. Then our reloaded app will correctly pull in any new
changes.

/cc @meastman