Add support for --allow-containers and --prefer-container

This PR:

• Adds support for --allow-containers and --prefer-container which will be used by DIRAC
• Adds some very simple integration tests which currently cover
  • CMake Gaudi
  • CMT Gaudi
  • Running with and without containers
  • Running all of the above SLC 6, CentOS 7 and Debian with Python 2.7, 3.5, 3.6 and 3.7
• Fixes some Python 3 compatibility issues

Supersedes: !76 (closed)

Requires:

• lhcb-docker/python-deployment!11 (merged) (I'm currently using containers from my fork)
• LbPlatformUtils!43 (merged) (Currently pip installing from my fork)