This reverts commit 102ebb84.

Prevent this kind of errors when first installation fail:

    Error: [Errno 13] Permission denied: 'parts/python-language-server/bin/activate'

by using --clear the virtualenv is recreated from scratch if the command failed
for some reason.

include some good monospace fonts: source code pro and jetbrains mono

https://mariadb.com/kb/en/general-query-log/

https://mariadb.com/kb/en/compiling-mariadb-for-debugging/

https://mariadb.com/kb/en/enabling-core-dumps/

result: it does not seem to cresh

We can probably make a scalability test from this, this a first step to
reproduce the crash

This way buildout can reuse egg caches and it's a bit faster:

To run a simple instance buildout, from 2.837s it goes down to 1.875s.
To run slapos node instance 10 times just after requesting an ERP5 instance, it goes from ~112s to 98s

before
    hyperfine "/srv/slapgrid/slappart4/srv/slapos/inst/slappart0/tmp/shared/python2.7/60364a13cc977dd5a894e0239ac889b9/bin/python2.7 /srv/slapgrid/slappart4/srv/slapos/inst/slappart0/tmp/soft/c63ba7265399450b28f9ea6d5667a5e7/bin/buildout -U"
    Benchmark #1: /srv/slapgrid/slappart4/srv/slapos/inst/slappart0/tmp/shared/python2.7/60364a13cc977dd5a894e0239ac889b9/bin/python2.7 /srv/slapgrid/slappart4/srv/slapos/inst/slappart0/tmp/soft/c63ba7265399450b28f9ea6d5667a5e7/bin/buildout -U
      Time (mean ± σ):      2.837 s ±  0.275 s    [User: 2.481 s, System: 0.285 s]
      Range (min … max):    2.482 s …  3.222 s    10 runs

after
    hyperfine "/srv/slapgrid/slappart4/srv/slapos/inst/slappart0/tmp/shared/python2.7/60364a13cc977dd5a894e0239ac889b9/bin/python2.7 /srv/slapgrid/slappart4/srv/slapos/inst/slappart0/tmp/soft/c63ba7265399450b28f9ea6d5667a5e7/bin/buildout -U"
    Benchmark #1: /srv/slapgrid/slappart4/srv/slapos/inst/slappart0/tmp/shared/python2.7/60364a13cc977dd5a894e0239ac889b9/bin/python2.7 /srv/slapgrid/slappart4/srv/slapos/inst/slappart0/tmp/soft/c63ba7265399450b28f9ea6d5667a5e7/bin/buildout -U
      Time (mean ± σ):      1.875 s ±  0.067 s    [User: 1.660 s, System: 0.148 s]
      Range (min … max):    1.816 s …  2.038 s    10 runs

test will be much slower, but having two slaprunner compiling at the same time
in the same folder is not something supported.

By copy paste mistake wrong log files were configured for log rotation,
resulting with damaging logrotate for the whole partition.

See merge request nexedi/slapos!791

By using nginx it's possible to set it up to expose logs nicely with the
real frontend.

furl is used to rewrite URL from the frontend to add proper username and
password information.

Also use default OpenSSL version.

pyflaked

See merge request nexedi/slapos!807