package_manager.py: set dnf's releasever setting from DISTRO_CODENAME
So that:
1) dnf does not complain anymore about releasever not being set and then fail
for the same reason;
2) it's possible to refer to $releasever in dnf package feed configuration
(repo paths in particular) without hardconding the release name (pyro, morty, etc.)
Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
