support structured data(list and dict) in glance metadata

Support structure data in glance metadata
The the moment glance metadata is simple key value pair. Thus structured data
(list and dict)

Why structure data is wanted?
For boot-from-volume support in nova, the information about block device
mapping is stored in image metadata which is stored as metadata of glance.
That is, which device of guest device is which nova volume/ephemeral devices.
So it's a list of dict.

Example of metadata
metadata = {'name': 'fake public image',
           'properties': {
               'mappings': [
                   {'device_name': '/dev/sda1',
                    'snapshot_id': 0x12345678,
                    'delete_on_termination': False},
                   {'device_name': '/dev/sda2',
                    'no_device': True}],
               'block_device_mapping': [
                        {'virtual': 'ami', 'device': 'sda1'},
                        {'virtual': 'root', 'device': '/dev/sda1'},

                        {'virtual': 'swap', 'device': 'sdb1'},
                        {'virtual': 'swap', 'device': 'sdb2'},

                        {'virtual': 'ephemeral0', 'device': 'sdc1'},
                        {'virtual': 'ephemeral1', 'device': 'sdc2'}]}}