Update docs to describe how to run test locally

/reviewed-on !705

Promise plugins are important part of the instantiated configuration, thus
have to be checked for changes.

This promise will check that certificate matches the key and will check
that it is renewed before 15 days before expiration.

For some cases on-renew is not needed, so allow empty one in case if it's
not provided.

and other probably useful information.

Upgrade Gitlab from version 8.17 to 9.5.10.

https://gitlab.com/gitlab-org/gitlab-foss/-/blob/v9.5.10/doc/install/installation.md

/reviewed-on nexedi/slapos!699

Previous gitlab version was 8.17.0, this commit upgrade gitlab to version 9.5.10,
and update all configuration to work with this version according to the installation procedure,
see: https://gitlab.com/gitlab-org/gitlab-foss/-/blob/v9.5.10/doc/install/installation.md

/reviewed-on nexedi/slapos!704

If pgdata already exists, then postgres recipe will not update or install anything when buildout will run. This result to no configuration or wrapper updated if configuration changes.

For example this change:  29d1ed68  the path of `posgtres bin` is now in shared folder, then impossible to run postgres service anymore because path was not updated by buildout in wrapper.

/reviewed-on nexedi/slapos!703

Since 10.4, it seems mysql_install_db does not take anymore
BASEDIR/lib/plugin as default value for plugin_dir.

Shamefully copied from neoppod/stress-testing SR.

Only TestSlave class and its subclasses are tested, as those are enough
complex scenarios: master partition, kedifa partition and frontend partition.

This allows to use monitor-setup-url correctly.

Instead of forcing to set monitor port in some cases, just generate them,
so it's possible to correctly instantiate caddy-frontend on one partition
scenario like in webrunner or tests.