trivial fixes to node.xml
node.xml: remove useless zfs_img.conf file

This file is not used any where... So we should not create it
node.xml: rocks sync user must be run after useradd
node.xml: fix error udev setup
node.xml: first take at partitioning with zfs and mirror

create a sample default partitioning script which does:
first 20GB to /
the remaining used by zfs in a pool called &vm_container_zpool;

If there are two disks, they are mirrored (both the / FS and the
zpool are mirrored).
usersguide: minor fixes mostly formatting
usersguide: added a table with all attributes

added a table with a list of all attribute used by the img-storage
roll to help the user understand them