Merge tag 'v0.15.3' into features/shared

CHANGELOG.md

lib/spack/llnl/util/lock.py

lib/spack/llnl/util/tty/__init__.py

lib/spack/llnl/util/tty/log.py

lib/spack/spack/__init__.py

lib/spack/spack/binary_distribution.py

lib/spack/spack/cmd/env.py

lib/spack/spack/compilers/__init__.py

lib/spack/spack/container/images.json

lib/spack/spack/environment.py

lib/spack/spack/fetch_strategy.py

lib/spack/spack/installer.py

lib/spack/spack/main.py

lib/spack/spack/operating_systems/cray_backend.py

lib/spack/spack/package.py

lib/spack/spack/platforms/cray.py

lib/spack/spack/schema/container.py

lib/spack/spack/stage.py

lib/spack/spack/test/cache_fetch.py

lib/spack/spack/test/cmd/env.py