Mainly to fix parallel build.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81712

Add functionality to wrapper to ask for an exclusive CPU core.

/reviewed-on nexedi/slapos!184

Helps developing valid software release descriptors.
Allows checking instance parameters and published values against a software
release's schema in addition to validating the schemas themselves against
slapos' schema.json and jsonschema's schemas.

Also, describe a bit more "serialisation" override mechanism: these
properties cannot be made required at schema level, but for any
software-type entry a serialisation must be provided.

it is required for DBD-mysql-4.0.42 with MariaDB 10.1.

This simplistic mail setup does not pass address as command line arguments,
so it is not subject to the vulnerability protected against by this option.

See https://jira.mariadb.org/browse/MDEV-12950 for API changes.

Because this does not seem to be enough:
https://github.com/perl5-dbi/DBD-mysql/pull/133

We should keep using MariaDB 10.1 for ERP5 as long as perl-DBD-mysql
is not fixed properly.

  The version 2.2.0 depends on PyNaCL which is not well configured currently on buildout.

@Tyagov @luke 

This MR adds the following to all Jupyter notebooks:
 - A warning when a user inputs a reference that has already been used in one of his old Jupyter notebooks. 
 - Some extra information when entering magics such as the remaining required magics.
 - Entering the reference MUST now be the last magic entered(otherwise the reference check doesn't work).
 - A small addition to the message at the top of the notebook for the command ```%notebook_set_title```

Also, do note that the edited kernel uses files from [this commit](nexedi/erp5@4a3e5173) from my erp5 repo, so the changes there must be available for the kernel to work. [This merge request](nexedi/erp5!283) addresses those changes.

Sample screenshot:
![Screenshot](/uploads/a34e0b70b991302539c572ad4344ba56/Screenshot.png)


/reviewed-on nexedi/slapos!177

/reviewed-on nexedi/slapos!157