straighetend the configs:
no more custom source packages
using xvfb for proview compilation (easier)
fix bug with pwr pre-build process killing itself

Future Cython+ work will use it.

Fix for #20200514-218C705 - \[testnode\] frontend for log access

Depends on nexedi/erp5!1304

See merge request nexedi/slapos!848

This fixes the nextcloud SR, which was broken by
commit 92779bf4
(mariadb is not a part anymore).

This fixes commit a62e5e7b.
See also commit 491e6e28.

And set it as log_frontend_url in testnode config

Just add the following 2 lines in a SR:

[mariadb]
location = ${mariadb-10.4:location}

It does not build with GCC 8.2

Going Go1.14.9 -> Go1.14.10 brings in compiler and runtime fixes
including fix for crash in garbage-collector due to race condition:

https://github.com/golang/go/issues/40642
https://github.com/golang/go/issues/40641

Tested on helloworld SR.

See merge request nexedi/slapos!844

validators.url is enough, even for Caddy, to check that URL is correct, and
caddy_backend_url_validator was introduced before validators.

Also calling an external command for each slave takes a lot of time.

Thanks to this other sections can directly reference them, and so they are
correctly created as needed, so linking section does not need update-command

The password is anyway present in the section itself, so it's eventual change
will result with reinstalling the section.

Also change a bit existing frontend_url to manage it the same way.

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