Modify existing tests to use the name option for instance disks. The
configuration options 'disk' and 'disk-growth' are merged into a single
'disks' option, which is a list of dictionaries with 'size', 'growth' and
'name' items.

Also, add 'instance-device-names' QA test, which tests addition, renaming
and removal of named devices.

Signed-off-by: Christos Stavrakakis <cstavr@grnet.gr>
Reviewed-by: Helga Velroyen <helgav@google.com>