admin switch: Allow switching just remote names

This is a followup to the previous commit; for the installation media
case we want to keep the current origin ref, and only switch remotes.

tests/test-admin-deploy-switch.sh

@@ -50,3 +50,7 @@ ostree admin --sysroot=sysroot status > status.txt
 assert_file_has_content status.txt anothertestos
 
 echo "ok switch remotes"
+
+ostree admin --sysroot=sysroot switch --os=testos testos:
+
+echo "ok switch remote only"