Update QA tests to check disk and NIC names

Update QA tests to check disk and NIC names

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>