ref(builder) switch to pre-receive hook
feat(contrib/coreos): add debug log generator
docs(releases): update with post-release "+git" procedures
Merge pull request #2227 from gabrtv/pre-receive

ref(builder): switch to pre-receive hook
Merge pull request #2238 from carmstrong/debug_script

feat(contrib/coreos): add debug log generator
Merge pull request #2253 from mboersma/update-release-docs

docs(releases): update with post-release "+git" procedures
ref(user-data): "make discovery-url" copies from user-data.example

This also adds contrib/coreos/user-data to .gitignore, so that local
changes to this essential file do not cause "git status" to mark the
repo as dirty.