#138 VPN better packaging - entrypoint debug and refactor

Merge after !93 (merged) though they should not interact.

• debug instabilities in container launch
  • main found cause was: launching wg set wg0 peer xxx command for an already declared peer (cannot redeclare the same peer) is useless and sometimes crashes the wg0 interface that does not exist anymore => only launch command if the peer is not yet declared
  • misc cause were: need to test errors for every file/vpn command and have consistent/predictable behaviour => now fail frankly when an error occurs, with explicit error message (instead of trying to continue, which results in half-done commands and uncoherent state)
• refactor entrypoint.sh
  • split with a entrypoint_functions.bash which contains the functions common to all container entrypoints. We currently duplicate the file in each build_files directory though, to keep things clear and avoid raising the containers' build directory to $FEDBIOMED_DIR/envs/vpn/docker for all containers (instead of $FEDBIOMED_DIR/envs/vpn/docker/xxx/build_files
• misc: rename entrypoint.sh to entrypoint.bash for all dev and vpn containers