Add default certificates for mrstub

  Add default certificates for mrstub

    The default certificates are in folder /usr/src/app/cert
    The default certificates can be overriden by mounting a volume in 'docker run' command or in a docker-compose.yml file
    ex:
    in 'docker run':
    docker run -it -p 3905:3905 -p 3906:3906 -v "$PWD/certificate:/usr/src/app/cert" mrstub
    in 'docker-compose.yml':
    volumes:
          - ./certificate:/usr/src/app/cert:ro

Issue-ID: NONRTRIC-208
Signed-off-by: ecaiyanlinux <martin.c.yan@est.tech>
Change-Id: Id903fb745736e053984bc71061e3c1f2d58fafde