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>

- vyos-config: Proxy charm to configure Vyos Router PNF
- magmagw: Proxy charm to configure Magma AGW
- enodeb: Native charm to add day-2 operations in EnodeB