Unify the setting up script for integration tests and for the README

While setting up my own dak and then running integration tests I realized that the setup script for integration tests followed the setup/README almost to the letter. In this commit I've added a dak-setup.sh file that does what the integration_test/setup script used to do, but in a generalized way that also allows the user to run this when creating a local dak.

I've verified that integration_tests still pass.