Distribution EOLE

Target minions with grains in addition to all Windows machines

As a transition, we continue to join and install Veyon to all Windows
machines but now we add support for grains target.

Ref: #30726
Ref: #30727

Veyon: username as LDAP DN to work nicely with Python2 minions

When the minion is in Python2, the serialization may cause problem
with backslash.

• tmpl/eole-workstation.sls: declare Veyon username as an LDAP DN.

Ref: #30727

Veyon: install and configure for GNU/Linux systems

• README.rst: documentation for Veyon upstream repository management.

• eole-workstation/defaults.yaml: rework the eole-workstation:veyon to
  permit the configuration of an upstream repository.

• eole-workstation/osfamilymap.yaml: update structure to declare Veyon
  package name.

• eole-workstation/osfingermap.yaml: declare mandatory upstream
  repositories for Ubuntu Xenial and Bionic and an optional one
  for Focal.

• eole-workstation/libkwargs.jinja: format a dictionary to be used as
  salt function options (mostly pkgrepo.managed)

• eole-workstation/veyon/repo/init.sls: entry point to add the Veyon
  repository.

• eole-workstation/veyon/repo/install.sls: add the Veyon repository if
  configured.

• eole-workstation/veyon/repo/clean.sls: remove the Veyon repository
  if configured

• eole-workstation/veyon/package/install.sls: requires the upstream
  Veyon repository if configured.

• eole-workstation/veyon/package/clean.sls: require the clean of
  upstream Veyon repository if configured.

• eole-workstation/veyon/config/import.sls: fix import command to
  accept GNU/Linux and Windows variable syntax.

• eole-workstation/veyon/config/file.sls: serialise the veyon context
  to avoid encoding issue with python2 minions.

Ref: #30727

Update changelog and VERSION for 2.7.0

Ref: #30726
Ref: #30727