Some rolling release distros, such as Arch Linux, don't include a
VERSION_ID field in their os-release file.
Change release_dict_osr() to better handle this optional field
being absent.
Further improve the resilience of the release_dict_*() methods by
always returning a dict and using dict.get() in distro_identifier()
to supply a default, empty string, value when then key is missing.
Signed-off-by: Joshua Lock <joshua.g.lock@intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
