Add DNF_ALLOW_DOWNGRADE for install and downgrade operations
Add DNF_ALLOW_DOWNGRADE for install and downgrade operations

DNF_ALLOW_DOWNGRADE is used to add SOLVER_FLAG_ALLOW_DOWNGRADE for job. Then it
will result that in all cases of install and downgrade, dependencies can be
downgraded. SOLVER_FLAG_ALLOW_DOWNGRADE is better option then allow_erasing
that also adds possibility to remove packages.

This new behavior should provide better user experiences.

Closes: #293
Approved by: ignatenkobrain

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