chore(*): bump to CoreOS 444.0.0; fleet 0.8.1

https://coreos.com/releases/#444.0.0
Merge pull request #1880 from carmstrong/coreos_444.0.0

chore(*): bump to CoreOS 444.0.0; fleet 0.8.1
refactor(makefile): remove fleet, switch to deisctl and custom images
fix(user-data): restore default discovery url
docs(readme): update root readme with new deisctl instructions
docs(hacking): new docs for hacking on deis
fix(readme): address feedback, fix typos
fix(makefile): default to 3 nodes, test for docker binary
docs(readme): add hack instructions to readme, remove newline
ref(Makefiles): use $BUILD_TAG, which defaults to "git rev-parse"
ref(tests): use images from "make build" in component tests
fix(user-data): add fixed version of sed command to "make discovery-url"
fix(tests): improve functional test setup logic
feat(tests): add end-to-end acceptance test script for vagrant
docs(*): update documentation for new deisctl provisioning workflow