[OpenStack-I18n] release notes POT file now contains branch info

Hi team, Small news on release note translations. Previously POT files of release notes did not contain branch information (such as ocata, newton). I made a small change on reno (release note tool) and POT files of release notes now contains branch information (example [1]). Thanks, Akihiro [1] http://tarballs.openstack.org/translation-source/horizon/master/releasenotes...