Add container detection

See https://its.cern.ch/jira/browse/LBCORE-1653

With these changes, on my Ubuntu laptop, lb-describe-platform prints

LbPlatformUtils version: 1.1.3.dev16+g0aa09e4.d20181018
dirac_platform: x86_64-unknown.avx2+fma
host_os: x86_64-ubuntu1810
host_binary_tag: x86_64-ubuntu1810-gcc8-opt
os_id: ubuntu1810
arch: x86_64
model: Intel(R) Core(TM) i7-7560U CPU @ 2.40GHz
compatible_binary_tags:
singularity:
  /cvmfs/cernvm-prod.cern.ch/cvm4:
    - x86_64-centos7-gcc49-dbg
    - x86_64-centos7-gcc49-opt
    - x86_64-centos7-gcc62-dbg
    - x86_64-centos7-gcc62-do0
    - x86_64-centos7-gcc62-opt
    - x86_64-centos7-gcc7-dbg
    - x86_64-centos7-gcc7-do0
    - x86_64-centos7-gcc7-opt
    - x86_64-slc6-gcc46-dbg
    - x86_64-slc6-gcc46-opt
    - x86_64-slc6-gcc48-dbg
    - x86_64-slc6-gcc48-do0
    - x86_64-slc6-gcc48-opt
    - x86_64-slc6-gcc49-dbg
    - x86_64-slc6-gcc49-do0
    - x86_64-slc6-gcc49-op
    - x86_64-slc6-gcc49-opt
    - x86_64-slc6-gcc62-dbg
    - x86_64-slc6-gcc62-do0
    - x86_64-slc6-gcc62-opt
    - x86_64-slc6-gcc7-dbg
    - x86_64-slc6-gcc7-opt
    - x86_64+avx-centos7-gcc62-opt+o3
    - x86_64+avx2+fma-centos7-gcc62-dbg
    - x86_64+avx2+fma-centos7-gcc62-opt
    - x86_64+avx2+fma-centos7-gcc7-opt
  /cvmfs/cernvm-prod.cern.ch/cvm3:
    - i686-slc5-gcc43-dbg
    - i686-slc5-gcc43-opt
    - x86_64-slc5-gcc43-dbg
    - x86_64-slc5-gcc43-opt
    - x86_64-slc5-gcc46-dbg
    - x86_64-slc5-gcc46-opt
    - x86_64-slc5-icc11-dbg
    - x86_64-slc5-icc11-opt
    - x86_64-slc6-gcc46-dbg
    - x86_64-slc6-gcc46-opt
    - x86_64-slc6-gcc48-dbg
    - x86_64-slc6-gcc48-do0
    - x86_64-slc6-gcc48-opt
    - x86_64-slc6-gcc49-dbg
    - x86_64-slc6-gcc49-do0
    - x86_64-slc6-gcc49-op
    - x86_64-slc6-gcc49-opt
    - x86_64-slc6-gcc62-dbg
    - x86_64-slc6-gcc62-do0
    - x86_64-slc6-gcc62-opt
    - x86_64-slc6-gcc7-dbg
    - x86_64-slc6-gcc7-opt

Meaning that I could run i686-slc5-gcc43-opt with singularity ... /cvmfs/cernvm-prod.cern.ch/cvm3.

Issues:

• I hardcode the list of singularity images with their OS (it would be nice to detect what an image can run)
• I'm not convinced of the output format