Nextcloud's redis is causing `AF_UNIX path too long` on test nodes.

The first commit to adapt this test to slapos.testing was incomplete.
Now that nextcloud is fixed, we saw a few mistakes and python3
incompatibilities that are fixed in this commit.

This reverts commit 4636f49f.

This reverts commit 97884c30.

Package like lm-sensors, which depends on the kernel, shall be provided
by the OS.

/reviewed-on nexedi/slapos!642

It is proven to work on Debian 10.

/reviewed-on nexedi/slapos!640

slapos node collect require lmsensors, so use those ones provied with slapos
component.

/reviewed-on nexedi/slapos!641

Otherwise it will be installed without our special part, using system
curl.

/reviewed-on nexedi/slapos!639

/reviewed-on nexedi/slapos!635

also drop test_nextcloud_promises as promises are now checked by
framework.

/reviewed-on nexedi/slapos!636

Prove that:

 * VNC is accessible
 * monitor-base-url on each partition is accessible with password extracted
   from the main partition

frontend-instance-guid and frontend-additional-instance-guid are used to
control where the websocket frontend shall end up.

Real frontend is always requested.

Additional frontend is requested only if frontend-additional-instance-guid is
not empty. This frontend is published as url-additional and can be used for
access to users from some restricted networks.

publish-early does not work in this case, and monitor password is stored
in buildout elsewhere anyway.

Update almost all tests to use nexedi/slapos.core!64 

Notable changes:
 * the testcase class is generated dynamically with `makeModuleSetUpAndTestCaseClass`
 * `self.slap` allow low level control of slapos
 * IP addresses are available as `_ipv6_address` and `_ipv4_address` class attributes
 * `getSupervisorRPCServer` no longer exist, instead use `self.slap.instance_supervisor_rpc` (as context manager)
 * now the framework takes care of running promises and instance step fail when instanciation fail or when promises are not satisfied (as a result, some tests checking promises are dropped)
 * test needs `slapos` and `supervisord` commands in PATH. It's important to have a recent enough `slapos` and not the `~/bin/slapos` from slaprunner which sets `$SLAPOS_CONFIGURATION` 

Following tests were not updated:
 * `caddy-frontend` as this will require more work, because this software uses shared partitions and maybe other things will need to be adapted in the tests. 
 * `slapos-master` as it is same as ERP5 and I thought maybe we can do better than duplicating code, so I left it as is for now.
 * `nextcloud` as this fail the `slapos node instance` step.

There are also a few style changes or small fixes to make pylint happy and some other fixes for problems with softwares and also a fix for seleniumrunner flaky test.

/reviewed-on nexedi/slapos!624