NPB containers enhancements

[gfieni]

PR overview:

• NPB source image:
  • Change base image to Ubuntu Xenial ;
  • Sources are now downloaded by Docker at build time.
• NPB mpi-build image:
  • Change default compiler from gfortran/gcc to mpif77/mpicc ;
  • Add compiler flags for 64-bit architecture and large address space/sizes of sections ;
  • Remove Fortran compiler flag that increase the integer bit size (breaks benchmark verification phase) ;
  • Add more flexible suite generation script ;
  • Clean Dockerfile.