Install extra package in docker inventory
    
    Use TEST_SUBJECTS to specify the extra packages.
    
    ex:
    
    export TEST_SUBJECTS="docker:docker.io/library/fedora:27 /home/fred/RPM/RPMS/x86_64/systemd-236-0.1.20171207151953.e97b7b5.fc27.x86_64.rpm  /home/fred/RPM/RPMS/x86_64/systemd-libs-236-0.1.20171207151953.e97b7b5.fc27.x86_64.rpm  /home/fred/RPM/RPMS/x86_64/systemd-pam-236-0.1.20171207151953.e97b7b5.fc27.x86_64.rpm"
    
    Fixes: #106