No longer index nodes by their name but by their UUID in the cluster
config. This change changes large parts of the code, as the following
adjustments were necessary:
 * Change the index key to UUID in the configuration and the
   ConfigWriter, including all methods.
 * Change all cross-references to nodes to use UUID's.
 * External interfaces (command line interface, IAllocator interface,
   hook scripts, etc.) are kept stable.
 * RPC-calls can resolve UUID's as target node arguments, if the RPC
   runner is based on a ConfigWriter instance. The result dictionary is
   presented in the form the nodes are addressed: by UUID if UUID's were
   given, or by name if names were given.
 * Node UUID's are resolved in ExpandNames and then stored in the
   OpCode. This allows to check for node renames if the OpCode is
   reloaded after a cluster restart. This check is currently only done
   for single node parameters.
 * Variable names are renamed to follow the following pattern:
   - Suffix is 'node' or 'nodes': Variable holds Node objects
   - Suffix is 'name' or 'names': Variable holds node names
   - Suffix is 'uuid' or 'uuids': Variable holds node UUID's
 * Tests are adapted.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>