Investigate including policycoreutils as a runtime dependency in order to be able to use semanage

Closes #1485 (closed).

This change replaces chcon with semanage in the gitlab-shell recipe, which makes the fcontext change permanent and brings the setup more in line with current SELinux best practices. As a result, the security context shouldn't get messed up anymore.

The documentation and test descriptions have been updated to reflect the change.

This change requires policycoreutils-python as a runtime dependency. The sort of counterintuitively-named restorecon -R -v is necessary to apply the new ssh_home_t context.

The dependencies required are in the base repos for all supported RHEL flavors.