Second step towards multiarch myplc https://svn.planet-lab.org/ticket/214

Second step towards multiarch myplc https://svn.planet-lab.org/ticket/214
-- design
* new notion of a 'nodefamily' - that for now is <pldistro>-<arch>
* standard rpm repository is now named /var/www/html/install-rpms/<nodefamily>
  a legacy symlink install-rpms/planetlab is created my myplc packaging
* bootstrafs rpm name is now bootstrapfs-<nodefamily> so that several instances
 from various builds can be installed in the same myplc
* noderepo rpm (named noderepo-<nodefamily>) built as part of the planetlab distro
  this is for installing /var/www/html/intall-rpms/<nodefamily> from another build

-- other changes
* extensive cleanup of mkfedora:
** obsolete  options -p -g -x removed
** option -p removed, pkgsfile is a mandatory argument
** option -k removed, use kexclude: in the pkgs file instead
* also cleaned up pl_root_* functions - no wrapper to pl_root_mkfedora anymore

-- status
* not tested at all - likely to break
* next step planned for storing nodefamily and pass it to yum.php.conf
  (impacted would be bootmanager, nodemanager and nodeconfig)
* a final cleanup in mkfedora would be to define mirrors in a separate .pkgs-like file
* kupdate.sh, as well as possibly other convenience scripts, might be impacted