Fixes and improvements

User-facing changes:

• Fix check for modified Gaudi/Makefile ("Could not remove Gaudi/Makefile safely!" error)
• Fix passing of environment variables in build-env
• Fix *-clean targets
• Fix passing of ARGS to ctest. Now dollars don't need escaping, so make fast/Moore/test ARGS='-R test$' works

Internal changes:

• Update to cmake 3.15.1 and install it directly from release binary
• More elaborate CI
• Relax kerberos ticket requirement in build-env
• Add new gitBase setting (automatically defaults to ssh://git@gitlab.cern.ch:7999 if accessible)
• Check for cvmfs when using docker (mostly for macOS hosts)
• Refactor setup.py to have it usable as a module
• Produce python.env file needed for VS Code python integration