No longer index instances 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.
 * External interfaces (command line interface, IAllocator interface,
   hook scripts, etc.) are kept stable.
 * Instance UUID's are resolved in ExpandNames and then stored in the
   OpCode. This allows to check for instance renames if the OpCode is
   reloaded after a cluster restart. This check is currently only done
   for single instance parameters.
 * Instance locking unfortunately can't use instances UUID as
   identifiers. The reasons is that new instances (which have no UUID
   yet) have to be locked as well, so the instance name is used.
 * Variable names are renamed to follow the following pattern:
   - Suffix is 'inst' or 'insts': Variable holds Instance objects
   - Suffix is 'name' or 'names': Variable holds Instance names
   - Suffix is 'uuid' or 'uuids': Variable holds Instance UUID's
 * Tests are adapted.

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