Fix up slave-setup.sh to use setup_fns.sh too.

Summary:
(I missed this one before because it's called slave-setup.sh, not
setup.sh.)

I moved some code that's shared between jenkins and jenkins-slave into
setup_fns.sh, notably around preparing a machine to build webapp (a
subset of what khan-dotfiles does).

I took out the code in the machine setup that installed ruby packages
directly.  Instead, we now depend on the webapp Makefile to install
the ruby deps, just like we do for python deps/etc.

Finally, I resolved some TODOs, because why not?

Test Plan:
Not tested.  I don't have cause to make a new jenkins-slave AMI, but
will test this next time I do!

Reviewers: alpert, chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D13679