Add constants for storage types to constants.py

This patch adds the currently known and used storage methods to the
constants.py file. Note that storage types are different from disk
templates. For example both disk templates, drbd and plain, use lvm
storage types.

The rapi documentation asserts that valid storage types are limited
to lvm and file. With the extension of storage types, this is not true
anymore, but the implementation of listing storage units is not
implemented for the new types. We disable the assert for now, adding
a fixme to extend it once the listing of storage units is extended
for the new types as well.

'gnt-node list-storage' currently does not support any other storage
type than 'lvm' and 'file'. We limit the QA to storage types that are
currently implemented for this operation.

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