Make modifyrepo insert size and open-size tag

Make modifyrepo insert size and open-size tag

Because this createrepo tool is quite old,
modifyrepo does not insert size and open-size tag in 'repomd.xml'.

This patch makes modifyrepo insert size and open-size tag
to avoid unexpected error in libzypp package.

To be honest, createrepo insert size and open-size tag in same version.

Applying this patch makes createrepo and modifyrepo
insert same tags, eventually.

Change-Id: Id0d4389878fc9880ab209caf3a22646a32dbae5e