refactor settings for install area and make them config options

etc/spack/install.yaml

lib/spack/spack/__init__.py

lib/spack/spack/build_environment.py

lib/spack/spack/cmd/__init__.py

lib/spack/spack/cmd/common/arguments.py

lib/spack/spack/cmd/deactivate.py

lib/spack/spack/cmd/extensions.py

lib/spack/spack/cmd/find.py

lib/spack/spack/cmd/reindex.py

lib/spack/spack/cmd/uninstall.py

lib/spack/spack/config.py

lib/spack/spack/database.py

lib/spack/spack/directory_layout.py

lib/spack/spack/install_area.py

lib/spack/spack/package.py

lib/spack/spack/repository.py

lib/spack/spack/schema/install.py

lib/spack/spack/spec.py

lib/spack/spack/test/cmd/uninstall.py

lib/spack/spack/test/database.py