Provide "config only" option on repository path element

[jberry-suse]

openSUSE letter staging projects are setup to include target project config without building against target project using a rather ugly hack. Since the "ports" repository exists (even though no longer used) and it builds archs not built in staging it effectively only adds the prjconf via that element and no binaries.

For example, see openSUSE:Factory:Staging

<project name="openSUSE:Factory:Staging">
  <!-- ... -->
  <repository name="standard">
    <path project="openSUSE:Factory" repository="ports"/>
  </repository>
</project>

This creates problems for tools like repo-checker which needs to expand the path chain, but should not expand the config-only path. Currently a special case is introduced, but really this should be expressed in OBS. It is rather convoluted to handle in tools since ports want to link to ports and SLE actually references the real target project stacks recursively. Further complex when trying to do a published and state check for all relevant archs before actually looping over archs and checking.

Something along the lines of:

<project name="openSUSE:Factory:Staging">
  <!-- ... -->
  <repository name="standard">
    <path project="openSUSE:Factory" repository="standard" prjconfonly="true"/>
  </repository>
</project>

Feel free to take liberties on the specific attribute name.