Before this commit, instance parameters that have a default null value
in JSON format were rendered as 'None' string in instance-theia.cfg.

In particular, Theia frontends were always requested with unsolvable
SLA parameters that prevented a frontend from ever being allocated:
```
sla-instance_guid = None
```

proxy.config.snapshot_dir is unavailable since TrafficServer 8.

See 9c5d25cb; sorry for missing to update the duplicate in that patch.

In commit 48b24182 I passed 'wcfs_enable' to instance-zope.cfg as string
instead of an object. As the result, even if it was wcfs_enable=False, in
instance-zope.cfg in

    ---- 8< ---- https://lab.nexedi.com/nexedi/slapos/blob/8d3af101/stack/erp5/instance-zope.cfg.in#L68-72)
    {% if slapparameter_dict['wcfs_enable'] %}
      WENDELIN_CORE_VIRTMEM=r:wcfs+w:uvmm
    {% else %}
      WENDELIN_CORE_VIRTMEM=rw:uvmm
    {% endif %}

it was always taking the first branch, becuse slapparameter_dict['wcfs_enable']
was giving 'False' - a string - whose boolean value is True

    In [1]: bool('False')
    Out[1]: True

-> Fix it by passing slapparameter_dict['wcfs_enable'], similarly to most other
keys, as object.

Due to the typo in 6572375c the revision was not unset and so it was
wendelin.core 1 that was still being built and installed.

All data related to the instance shall be in the instance itself, and as
temporary files are crucial for caucase, force the macro user to set the
location.

This is required for example to preview images (ie. open a .png file from the
editor). Without the default value of {{uuid}}.mini-browser.{{hostname}} is used
but we don't have anything in place to resolve such domain names.

See merge request nexedi/slapos!990

Going Go1.15.11 -> Go1.15.12 brings in fixes to compiler, runtime and stdlib
including security fix to net/http package:

https://golang.org/doc/devel/release.html#go1.15.minor

Tested on helloworld SR (by adjusting it to use go1.15 instead of go1.16).

Going Go1.16.3 -> Go1.16.4 brings in fixes to compiler, runtime and stdlib
including security fix to net/http package:

https://golang.org/doc/devel/release.html#go1.16.minor

Tested on helloworld SR.

Before this commit, the Go workspace was always activated before
launching the Theia shell and starting the embedded supervisord.

This in turn added the gcc installed by Theia to the PATH, meaning
that nested instances would be compiled with Theia's gcc instead of
the system gcc, making the compilation outcome dependant on Theia.

With this commit, the gcc resolved by PATH is now the system one.

See merge request nexedi/slapos!987

This option takes a string representing a valid JSON object and
forwards the corresponding python dict to the embedded instance.

Features:

 * random response is possible of big sizes
 * timeout can be randomized

See merge request nexedi/slapos!979

This fixes 157dd7ab.

It workarounds stack/resilient/pbsready.cfg.in directory:backup string
concatenation:

  backup = $${basedirectory:backup}/$${slap-parameter:namebase}

See merge request nexedi/slapos!981

This fixes:

  Installing jupyter.
  While:
    Installing jupyter.
    Base installation request: ...
    Getting distribution for 'argon2-cffi'.
  Error: Picked: argon2-cffi = 20.1.0

Addition of argon2-cffi in commit 7d1ea024
was a last-minute change to fix jupyter SR.