Index nodes by their UUID

Index nodes by their UUID

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>