Major rewrite of LHCb contextualization

based on the technique used in Eclipse CHE containers (see https://github.com/eclipse/che-dockerfiles/blob/master/recipes/stack-base/centos)

• use ENTRYPOINT instead of hooking into /etc/profile.d
• use parrot_run to provide in-container /cvmfs access (if needed)
• changed the communication protocol to the contextualization
  • use standard -u docker run option for USER_ID:GROUP_ID
  • environment variables USER/GROUP for custom user/group name
  • environment variable NO_LBLOGIN to avoid sourcing LbLogin
  • autodetect the need for in-container /cvmfs (-e NO_CVMFS=1 to disable)

Note: to mount /cvmfs in teh container docker run should be invoked with --privileged or --security-opt seccomp=unconfined.