Publish only for active slaves.

Empty extra_slave_instance_list is a leftover of never fully working approach
of slaveless caddy-frontend implementation, so just drop it.

See merge request nexedi/slapos!1280

In some contexts it's required to not stop buildout processing on publishing
errors, thus such recipe is required in those cases.

By providing -error-status-file it's possible to check status of the publish,
the file will be created on error and removed on successful call or during
uninstall. It's responsibility of the caller to handle such errors, for example
by using proper promises.

Introduce macro.pythonpath.egg to compute during install the PYTHONPATH
of setup eggs required to install other eggs, and insert the PYTHONPATH
into the environment section such as used by zc.recipe.egg:custom.

Both numpy and scipy use a Python subprocess to call Cython in their
setup.py, therefore the setup-eggs option of zc.recipe.egg:custom
does not work because sys.path is not propagated to the subprocess.

See merge request nexedi/slapos!1301

See nexedi/slapos@22cc8e7e

See merge request nexedi/slapos!1251

Includes buildout, configuration, test, and resiliency support

See merge request nexedi/slapos!1298

Allow ffmpeg to get the correct libvpx path

Using "Sort Lines Ascending" from Theia or `:'<,'>!sort -f` from vim
produce this diff.

See merge request nexedi/slapos!1279

See merge request !1295