pollster/api sample notifications should have dictionary payload

the rough format of api/pollsters notificatoins is as follows:

NOTIFICATION = {
    u'_context_domain': None,
    u'_context_request_id': u'req-da91b4bf-d2b5-43ae-8b66-c7752e72726d',
    'event_type': u'sample.create',
    'timestamp': u'2015-06-1909: 19: 35.786893',
    u'_context_auth_token': None,
    u'_context_read_only': False,
    'payload': [{
                u'counter_name': u'instance100',
                u'user_id': u'e1d870e51c7340cb9d555b15cbfcaec2',
                u'resource_id': u'instance',
                u'timestamp': u'2015-06-19T09: 19: 35.785330',
                u'message_signature': u'fake_signature1',
                u'resource_metadata': {u'foo': u'bar'},
                u'source': u'30be1fc9a03c4e94ab05c403a8a377f2: openstack',
                u'counter_unit': u'instance',
                u'counter_volume': 1.0,
                u'project_id': u'30be1fc9a03c4e94ab05c403a8a377f2',
                u'message_id': u'4d865c6e-1664-11e5-9d41-0819a6cff905',
                u'counter_type': u'gauge'
                },
                {
                u'counter_name': u'instance100',
                u'user_id': u'e1d870e51c7340cb9d555b15cbfcaec2',
                u'resource_id': u'instance',
                u'timestamp': u'2015-06-19T09: 19: 35.785330',
                u'message_signature': u'fake_signature12',
                u'resource_metadata': {u'foo': u'bar'},
                u'source': u'30be1fc9a03c4e94ab05c403a8a377f2: openstack',
                u'counter_unit': u'instance',
                u'counter_volume': 1.0,
                u'project_id': u'30be1fc9a03c4e94ab05c403a8a377f2',
                u'message_id': u'4d866da8-1664-11e5-9d41-0819a6cff905',
                u'counter_type': u'gauge'
                }],
    u'_context_resource_uuid': None,
    u'_context_user_identity': u'fake_user_identity---',
    u'_context_show_deleted': False,
    u'_context_tenant': u'30be1fc9a03c4e94ab05c403a8a377f2',
    'priority': 'info',
    u'_context_is_admin': True,
    u'_context_project_domain': None,
    u'_context_user': u'e1d870e51c7340cb9d555b15cbfcaec2',
    u'_context_user_domain': None,
    'publisher_id': u'ceilometer.api',
    'message_id': u'939823de-c242-45a2-a399-083f4d6a8c3e'
}

since payload is a list, it does not make it very extensible. for us to send anything else, we need to create an entire different event.