libostree: Reformat bootloader titles

Use the pattern:

  $PRETTY_NAME [$COMMIT_VERSION] (ostree[:$OSNAME][:$DEPLOYMENT_INDEX])

$OSNAME is only shown if there are multiple values.

$COMMIT_VERSION refers to the version tag in the commit's metadata.

$DEPLOYMENT_INDEX is only shown if no $COMMIT_VERSION is available.

https://bugzilla.gnome.org/show_bug.cgi?id=739416

tests/admin-test.sh

@@ -29,6 +29,7 @@ ostree admin --sysroot=sysroot deploy --karg=root=LABEL=MOO --karg=quiet --os=te
 ostree admin --sysroot=sysroot status | tee status.txt
 
 assert_file_has_content status.txt 'Version: 1.0.10'
+assert_file_has_content sysroot/boot/loader/syslinux.conf 'TestOS 1.0.10'
 
 echo "ok deploy command"