support for multiple slice reference images

support for multiple slice reference images

* vserver-reference rpm names have changed and become
    vserver-@pldistro@-@fcdistro@-@arch@
    vserver-systemslices-@pldistro@-@fcdistro@-@arch@

* this contains the images located as
   /vservers/.vref/@pldistro@-@fcdistro@-@arch@
and
  /vservers/.vstub/@pldistro@-@fcdistro@-@arch@/planetflow
respectively

* a stamp gets created on the nodes as /etc/planetlab/defaultvref
  that contains the name for the default slicefamily to use

* nodemanager tries to be smart when chosing the right argument to pass to vuseradd
  for this purpose, the argument in 'vref' is taken as a dash-separated list of 'wishes'
  so e.g. is defaultvref is planetlab-f8-x86_64, then we should find for the image name:
vref=i386           -> vuseradd -t planetlab-f8-i386
vref=centos5        -> vuseradd -t planetlab-centos5-x86_64
vref=planetflow     -> vuseradd -t planetflow
vref=centos5-onelab -> vuseradd -t onelab-centos5-x86_64

for legacy reasons, if the stamp is not found, 'default' is taken instead

=== status
* this is only partly tested
* a given node can install several variants of vserver-*
  this would require a manual upload of the rpms into the right install-rpms
  and manual changes in yumgroups.
* next improvement would be to have the system (planetflow) slices have a smarter naming scheme as well
  this way a 'planetflow' vref would rather be rewritten as planetflow-f8-x86_64
  for the time being it is not supported to install several variants of vserver-systemslices
  on the node - although it should be harmless