Check for MCR related env vars when instantiating SPMCommand

The recently added support for special environment variables, that allow
enforcing of the standalone (mcr) version of SPM, was incomplete. It only
affected requests for SPM's version information. With this patch, the
environment variables are also checked when instantiating SPMCommand.
Merge pull request #780 from rku/enh/spmmcr_env_config

Check for MCR related env vars when instantiating SPMCommand. coverage is good: https://coveralls.io/files/126696593#L250