Optional support for GPU sharing with CUDA MPS

NVIDIA offers a proxy process that coalesces CUDA commands from multiple processes for concurrent kernel execution, called [MPS (Multi-Process Service)](https://docs.nvidia.com/deploy/pdf/CUDA_Multi_Process_Service_Overview.pdf).However, it requires `--ipc=host` option for nvidia-docker setups (NVIDIA/nvidia-docker#419) which may compromise security in multi-tenant setups like Backend.AI.

Let's keep track of how this technology is going to evolve and apply to Backend.AI when appropriate.

First we could make it an opt-in feature so that our customers who use private Backend.AI clusters with semi-trusted users (e.g., employees in the same company) can benefit from performance improvements.

JIRA Issue: BA-333