Signed-off-by: beierlm <mark.beierl@canonical.com>

Related change in IM: https://osm.etsi.org/gerrit/#/c/osm/IM/+/11212/

A simple descriptor that uses a native charm to launch a clean
Ubuntu base image and layer a graphical desktop environment
and RDP server on top.

Signed-off-by: beierlm <mark.beierl@canonical.com>

Changes the charm from relying on a prepackaged image to
building srsue and srsenb from tagged source.
Updates the descriptor to use generic name for relation.

Signed-off-by: beierlm <mark.beierl@canonical.com>