Use SCL gem command when building SCL package

Addressing:
+ gem unpack /builddir/build/SOURCES/foreman_openscap-0.2.0.gem
/var/tmp/rpm-tmp.J7fM2a: line 31: gem: command not found
UI parts to assign hosts with compliance policy

Relates to #9
Depends on theforeman/foreman issue #8592.

WIP: only brings the UI parts.
Internal server error in installation within katelo.

Fixes #14.