Adjust gbp config for byzantium

debian/gbp.conf

 [DEFAULT]
+debian-branch=pureos/byzantium
+debian-tag = pureos/v%(version)s
+debian-tag-msg = %(pkg)s v%(version)s
 pristine-tar = True
-debian-branch = debian/master
 upstream-branch = upstream/latest
 upstream-vcs-tag = %(version)s
 
+[tag]
+sign-tags = True
+
 [buildpackage]
 sign-tags = True