Use a generated password and publish its url.

Remove

```
[gcc:python2]
part = gcc-8.5
```

from neoppod/software-common.cfg and dream/software.cfg as it causes
gcc being pinned to a different version before and after rebootstrap
when the initial python version is not Python2.

This reverts dc91e5d4.

When use-ipv6 is enabled, make mariadb publish an url of the form:
  `mysql://user:password@[ipv6]:port/database`

instead of:
  `mysql://user:password@ipv6:port/database`

which results in the zope instances crashing during processing
due to urlparse.urlsplit failing to parse the url.

Make it possible to mock any specific cron entry.

Those are candidates to be shared by some other place, like slapos testing.

See merge request nexedi/slapos!1693

Following advice from https://luajit.org/download.html use recent commit id,
but rely on github archive.

Drop obsoleted patch, enforce --with-shared-termcap-library

Also use directly archive with patches.

Changes:
 * use xz archive
 * stabilize patch name
 * update patch to current version