Update testing framework

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