New features:
- Use latest Xenial stemcell as default.
- Use cflinuxfs3 stack when pushing cf apps, if it is available.
- Upgrade BOSH CLI version in tile-generator docker image.
- Upgrade Flask in sample app.
Bug fixes:
- Pass placeholder values to `form_types` so they show up in Ops Manager forms.
- CI: specify a space when setting the cf cli target to avoid hangs.