_update_hash_filename_ key is used to avoid clash with recipe.

hexagonit.recipe.download is replaced with slapos.recipe.build:download in
order to have the same approach everywhere and so _update_hash_filename_ key is
used to avoid collision with section:filename.

Use _update_hash_filename_ to avoid filename clash with recipes.

_update_hash_filename_ is used to avoid conflict of section:filename key with
hexagonit.recipe.download

A lot recipes treats filename key as something special, so for them support
additional key _update_hash_filename_, in order to avoid such clash.

/reviewed-on nexedi/slapos!634

this is the firefox we are currently using on testnodes. It's useful to
be able to run test with exactly same version to debug issues.

/reviewed-on nexedi/slapos!646

Most important changes:

 * promise tests are dropped, as this is implemented by test case
 * slave is always instantiated, to pass promise checks
 * move out some methods from the class
 * change the way how supervisor is called
 * adapt requestDefaultInstance to request slaves

New test system requires a lot of work for testing some edge cases of
caddy-frontend instantiation, so skip some tests for now.

caddy-frontend cluster stabilises only in case if slaves are present, as
otherwise slapos node instance won't run w/o errors, so request slaves
early.

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 !639

/reviewed-on nexedi/slapos!635

also drop test_nextcloud_promises as promises are now checked by
framework.

/reviewed-on !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.