After escaping strings with dumps (to avoid buildout breakage and code
injection on "tricky" references, e.g. with spaces), promises started to be
generated with !py! prefix in their filenames, e.g. as

    '!py!'\''RU1-sdr-busy.py'\'''

instead of just

    RU1-sdr-busy.py

The issue here is that our code set

    name = !py!'RU1-sdr-busy.py'

and then monitor-promise-base from stack/monitor does

    output = ${directory:plugins}/${:name}

which, I though, would be expanded to .../etc/plugin/RU1-sdr-busy.py but it did
not deserialized the :name upon expansion.

I feel like it is maybe a bug in nexedi/slapos.buildout@4e13dcb9 , but I'm not sure.

Anyway, workaround the problem by generating :output ourselve also via escaped way.