-  add --nouse-image flags to cros_sdk to prevent usage of missing lvm2
-  add dynamic discovery of ebuild files instead of rely on hard-coded versions

/reviewed-on nexedi/slapos!219

/reviewed-on nexedi/slapos!220

JupyterLab is provided side-by-side with Jupyter Classic interface with same process.

Published URL changed:

 * jupyterlab-url provides IPv6 URL to JupterLab interface
 * jupyter-class-url provides IPv6 URL to the classic UI

Thanks to using only one process, the path in the URL differentiates the style of the interface.

Note: This MR replaces closed nexedi/slapos!207

/reviewed-on nexedi/slapos!218

component/gnutls: version up libgcrypt 1.8.1, including security fixes. also version up gnutls 3.5.15.

See !196

The whole code is small enough and having the template
in a separate file only made editing more annoying.

gcc was extended by python-2.7 before
commit 2bcd7326

This prevented from overriding 'erp5' without affecting the 'wendelin' itself.
Repository sections are often customized to disable cloning and make 'location'
point to an existing directory.

  If someone is developing an erp5 test node inside a webrunner, there is no reason to put entire data on resilience.

  When a file is removed while rsync is running (after build the list and before transfer the file), rsync fail with the follow message:

     rsync warning: some files vanished before they could be transferred

  This commit includes   --ignore-missing-args into rsync command line to ignore this case, as it is not relevant to the resilience
if a file is removed.

This parameter came from monitor work, but it is shadowing a parameters
already relied upon by ERP5 SR. Changing its value causes a regression
in ERP5 SR: node-id, which is supposed to be stable accros instance
trees, changed because of this.

This part is not invoked anymore.

software/slapos-master: fix slapos master to add parameters required for apachedex and mariadb promise

/reviewed-on !215

also update groonga 7.0.5 and mroonga-7.05.