stack/erp5 += WCFS service (technology preview)

This complements 7f877621 (Move wendelin.core from Wendelin to ERP5) and
introduces new WCFS service into ERP5 software-release to support
upcoming wendelin.core 2. Wendelin.core 2 will introduce new WCFS
filesystem server component that serves in-ZODB arrays data as OS files
for Zopes to memory-map. Please see the following links for overview of
what WCFS is and how it interoperates with and serves Python clients:

https://lab.nexedi.com/kirr/wendelin.core/blob/b5fc98bb/wcfs/wcfs.go#L20-247
https://lab.nexedi.com/kirr/wendelin.core/blob/b5fc98bb/wcfs/client/wcfs.h#L20-96

Implementation notes:

- WCFS is by default disabled in ERP5 - we do not want to load every
  ERP5 user with it in the early days of wendelin.core 2. WCFS service
  will also not work when ERP5 is built with wendelin.core 1, so keeping
  it disabled by default is reasonable for now.

- There is a duplication for instance-erp5.cfg.in in between stack/erp5/
  and software/slapos-master/ . It was negotiated with VIFIB developers
  to go this way instead of first deduplicating instance-erp5.cfg.in
  copying that was started in 87d13789 (slapos-master: Copy files from
  erp5 stack for local changes).

To enable wcfs one can either pass `wcfs` configuration parameter, or
add the following to a software-release that uses stack/erp5:

  [erp5-defaults]
  wcfs-enable-default = true

/cc @rafael, @tomo