cis-dil-benchmark-5.6: add check for users in wheel group

[schurzi]

Is your feature request related to a problem? Please describe.
In our current implementation, we check if a group wheel exists. This is not true for Debian based distributions and it is also not the intended check for the mentioned control.

Details see: https://secscan.acron.pl/centos7/5/6 or benchmark PDF from CIS.

Describe the solution you'd like
Implement a control, that verifies all users in group wheel with a user supplied list, of allowed users. By default only root should be allowed to be in this group. It is explicitly OK if this group does not exist.

Additional context
see #138