gnt-cluster modify/init: deprecate --no-lvm-storage

This patch does three things:

1. It deprecates the option '--no-lvm-storage' of 'gnt-cluster modify'
and 'gnt-cluster init'. Technically, it is not fully removed, but kept in order
to warn the user that it is no longer supported and that she should use
--enabled-disk-templates instead.

2. The consistency check between '--no-lvm-storage' and '--vg-name' is replaced
by checks between '--enabled-disk-templates' and '--vg-name'. There are these
cases:
- vg name, lvm disk template enabled = ok
- no vg name, lvm disk template enabled = error
- vg name, no lvm enabled = warning
- no vg name, no lvm enabled = ok
I added quite a lot of tests for all these and the transitions from each case
to another to the QA.

3. The check whether or not the volume group is available on all nodes is now
done only in these cases:
- the volume group name gets set and lvm is already enabled
- lvm is getting enabled and the volume group was set before

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>