tests(docs): add documentation tests to test-acceptance script

Also fixes a missing import in docs/docs_requirements.txt.
chore(*): remove fleet/systemd units

These are now maintained in deis/deisctl.
Merge pull request #1889 from mboersma/1888-make-docs-acceptance

tests(docs): add documentation tests to test-acceptance script
Merge pull request #1891 from carmstrong/remove_unit_files

chore(*): remove fleet/systemd units
feat(tests): add test-nightly.sh script for CI

To be used by our nightly buildpack / Dockerfile regression testing
jobs at http://ci.deis.io/view/example-apps/.
docs(contrib): update docs with deisctl
Merge pull request #1894 from mboersma/1893-nightly-tests

feat(tests): add test-nightly.sh script for CI
fix(tests): make test-nightly.sh executable
docs(releases): update release procedure with deisctl tasks
Merge pull request #1897 from bacongobbler/contrib-docs

docs(contrib): update docs with deisctl
Merge pull request #1899 from mboersma/update-release-checklist

docs(releases): update release procedure with deisctl tasks
ref(builder): use ubuntu:14.04 as Dockerfile base image
ref(cache): use ubuntu:14.04 as Dockerfile base image
ref(controller): use ubuntu:14.04 as Dockerfile base image
ref(database): use ubuntu:14.04 as Dockerfile base image
ref(database): update postgres user uid/gid in confd templates
ref(logger): use ubuntu:14.04 as Dockerfile base image
ref(registry): use ubuntu:14.04 as Dockerfile base image
ref(router): use ubuntu:14.04 as Dockerfile base image
ref(tests): use ubuntu:14.04 as Dockerfile base image