spec: do not enable RHSM plugin on CentOS
spec: do not enable RHSM plugin on CentOS

Signed-off-by: Igor Gnatenko <ignatenko@redhat.com>

Closes: #265
Approved by: jsilhan

edynox/libdnf,edynox/libdnf,edynox/libdnf,rpm-software-management/libhif,rpm-software-management/libhif,edynox/libdnf,rpm-software-management/libdnf,rpm-software-management/libhif,rpm-software-management/libhif,hughsie/libhif,Conan-Kudo/libhif,Conan-Kudo/libhif,hughsie/libhif,hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf,Conan-Kudo/libhif,hughsie/libhif,Conan-Kudo/libhif