Add support for Singularity containers

[mcopik]

Singularity is a containerization technology popular in the HPC. rFaaS should support running functions inside Singularity containers.

Tasks:

• [ ] Add switch for using singularity and Docker.
• [ ] Add JSON configuration (network, CPU affinity, memory).
• [ ] Add configuration of deployment of images.
• [ ] Invoke containers.
• [ ] Correctly handle the shutdown.