WIP: Purge CMT variables and improve build documentation

I think it's time to wave a fond farewell to all the legacy CMT variables that can (or need!) to be used to bootstrap the Gaudi build.

The old CMTCONFIG was already deprecated in favour of CMAKECONFIG, so I just systematically removed the CMT version.

The old CMTPROJECTPATH was still used to hint the location of the LCG release. Instead I have used the more standard CMAKE_PREFIX_PATH that seems to work from the environment or as a CMake -D option. However, I also added a fixed hint of /cvmfs/sft.cern.ch/lcg/releases that will actually work in most cases (CMAKE_PREFIX_PATH comes first, so can be used to point to non-standard places).

I updated the README in the cmake with what I could glean are actually the modern requirements (e.g., python 2.7 is needed these days). Bootstrapping directly from the LCG view is actually the easiest experiment neutral way to get going, AFAICS.

@clemenci probably needs to have a look for LHCb if the instructions make sense. For ATLAS my instructions do work, but @leggett and @bwynne should probably check I didn't miss a corner case.

Finally, I added a tiny top level README with links to other project resources.