[backends] zypp: Implement single-repo refresh feature

To get a list of available repositories, use:

    pkcon repo-list

To force-refresh a given repository, use:

    pkcon repo-set-data $REPOSITORY refresh-now true

To refresh a given repository if needed, use:

    pkcon repo-set-data $REPOSITORY refresh-now false

This feature has been implemented on top of repo-set-data, so that
we do not need to modify or add a new D-Bus API method to PackageKit.
Merge pull request #30 from thp/single-repo-refresh

[backends] zypp: Implement single-repo refresh feature