Update testing framework

Arndt, Danielrequested to merge
github/fork/dalg24/update_docker into master

Created by: dalg24

Following up on https://github.com/arborx/ArborX/issues/295#issuecomment-625326270

Scope creeped quite a bit...

List of changes:

  • Upgrade OpenMPI to v4
  • Allow non-CUDA enable MPI with CUDA build
  • Import gpg keys from local server
  • Update clang to v10
  • Add OMP_NUM_THREADS and MPI_MAX_NUMPROCS in testing
  • Replace one CUDA 10.1 build by 10.2
  • Reorganize backends distribution among builds

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