Option to whitelist/blacklist captured host's environment variables

[victorusu]

It would be nice if we could have an option to whitelist and/or blacklist the environment variables that are captured from the host's environment.

The use case is the use of application managers inside the container, where the identification of the system is done by environment variables, e.g. Spack identifies if the system is a Cray system by the existence of the environment variable CRAYPE_VERSION (https://spack.readthedocs.io/en/latest/getting_started.html#using-linux-containers-on-cray-machines). In this case, one has to unset the environment variable CRAYPE_VERSION before running spack with Sarus.