bcfg2-repo-validate doesn't handle including files from subdirs into clients.xml

[https://www.google.com/accounts/o8/id?id=AItOawlP6DRBvIJ7NLpLWILbgzURvGbMtGEyd8U]

In clients.xml, files from subdirectories can be included using XInclude. However, neither the schema nor bcfg2-repo-validate are set up to handle that:

$ cat Metadata/clients.xml
<Clients version='3.0' xmlns:xi='http://www.w3.org/2001/XInclude'>
  <xi:include href='subdir/foo.xml'/>
</Clients>

$ cat Metadata/subdir/foo.xml
<Clients>
  <Client name='one.zedat.fu-berlin.de' profile='server' pingable='Y'/>
  <Client name='two.zedat.fu-berlin.de' profile='server' pingable='Y'/>
</Clients>

$ bcfg2-repo-validate
/server/bcfg2/server/repository/Metadata/clients.xml ***FAILS*** to verify              <----
subdir/foo.xml:1: element Clients: Schemas validity error : Element 'Clients', attribute '{http://www.w3.org/XML/1998/namespace}base': The attribute '{http://www.w3.org/XML/1998/namespace}base' is not allowed.
subdir/foo.xml:3: element Client: Schemas validity error : Element 'Client': This element is not expected.
/server/bcfg2/server/repository/Metadata/clients.xml fails to validate

[https://www.google.com/accounts/o8/id?id=AItOawlP6DRBvIJ7NLpLWILbgzURvGbMtGEyd8U]

The new bcfg2-lint(8) tool handles XIncludes just fine, so this ticket can be closed.