deploy/ci: Some changes wrt disk sizes

vcluster.conf:

  disk0_size is the size of base vcluster image to debootstrap
  disk1_size if the size of the extra disk to use for LVM

ganeti.conf:

  vg_size is the size of the VG to create if an extra disk is not
found

Ganeti has a hard requirement for VG more than 20480MB.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>