Parameter to clear all environment variables

In our use case, we have a full environment defined for a container. We want to define only those and we do want to inherit any environment variable from host.

The current way to do it is to add `--unsetenv` parameters for every environment variable from host, before `--setenv` parameters.

This use case should be frequent enough for a `--clearenv` parameter to exist. This parameter would remove all environment variables. The command line would not need to depend on host environment variables.