Use unreal address to avoid any tries for network connectivity.

Kedifa requires some time to process new slave, and during that time the key
download URL is not available, but as it is required for proper mapping file,
use some url to mimic it.

Recent setuptools found a way to install by downloading wheels from PyPI.
But we want to control the versions of all components.

Buildout's kedifa updater just prepares, and so real one has to be run.

During buildout run no network communication is required in order to prepare
fallback certificates, so call kedifa-updater with --prepare-only

It is needed by users to check certificate of KeDiFa while uploading
certificates.

Each time new slave appears the kedifa-updater has to be run immediately, in
order for certificates to be properly setup.

Otherwise caddy can be left in non-runnable state until next kedifa-updater
would run again.

Improvements:

 * report processed file
 * process all files even if one is incorrect
 * report errors with exit code

/reviewed-on !558

Add missing keys and fix types.

URLs to generate authentication and to upload keys uses self-signed
certificates, which can't be verified.

caddy-frontend master partition does not implement any promise in etc/promise,
all is migrated to etc/plugin

is-process-older-than-dependency-set is not needed, since monitor stack does
not require the older-than-dependecy-set promise.

promise-monitor-httpd-is-process-older-than-dependency-set promise is not
needed anymore, as hash-files are used.

caddy-frontend-is-running-actual-software-release promise is not needed
anymore, as hash-files are used.

/cc @tomo

/reviewed-on nexedi/slapos!555

@rafael I think it would be useful if people can request an NBD server to hold custom image. What do you think ?

/reviewed-on nexedi/slapos!552