commands: remove nas name table when deleting hosts from DB

We need to remove the subtable used to store the nas hostname
before we can proceed when removing a host
commands: precache img-storage nas name when querying
imgstorage: fix bug with broken DB connection

if the database connection goes down the slqlight DB remains in
corrupted state, which does not allow removal of mapping.
Avoid this situation with a try.
node.xml: install iscsi daemon on FE

so we can run virtual fe using the iscsi mapping on the physical FE
node.xml: fix restart script after first boot
usersguide: first hint of a userguide

Written only few paragraph. It is still missing a lot of stuffs
commands: support hybrid VM disk image

now virtual machine can have both local file disk image as well
well remote iscsi based disk image.

If the set host vm nas value is not set for a given virtual machine
it will booted using the backing storage specified in its original
configuration (rocks list host vm showdisks=1).
Once the vm nas values is set, the iSCSI solution will be used,
and the original configuration will be ignored except for the size.
commands: add rocks dump to backup nas information

add rocks dump host vm nas to backup and restore values for vm nas
usersguide: finally looks decent
remove USING.md since it is now included in the usersguide
first version of userguide in rst
commands: fix identation

use tab no spaces
usersguide: some extra fixes

Finally the first usersguide in RST
removing old docbook userguide
Merge commit 'b6b73950a2515823f97c2235606e95d01ee5e1ab' into feature/sync_queue

Conflicts:
	USING.md