cross-module change for multi-arch myplc

cross-module change for multi-arch myplc

* name of the bootstrap tarball changed into
  bootstrapfs-<id>-<arch>.tar.bz2
    with id=<planetlab-distro> for the main tarball
    and  id=<extension> for the extensions tarballs

* bootmanager to use node's nodegroups to figure the tarball to use

* the noderepo rpm allows to install /var/www/html/install-rpms/<repo>
==
status
* checkpoint version to verify that mono-arch mono-distro still works
* build phase tested
* but some parts are still missing for complete multi-arch behaviour:
* bootstrapfs's rpm name should sontain pldistro and arch (as it is a noarch rpm)
* yum.php.conf also needs tunings