Python 3: fix a lot of tests

Python 3: fix a lot of tests

As of this commit, the following tests should now be working with Python 3:

- cinder.tests.unit.api.contrib.test_cgsnapshots
- cinder.tests.unit.api.contrib.test_scheduler_hints
- cinder.tests.unit.api.contrib.test_snapshot_actions
- cinder.tests.unit.api.contrib.test_snapshot_manage
- cinder.tests.unit.api.contrib.test_snapshot_unmanage
- cinder.tests.unit.api.contrib.test_volume_encryption_metadata
- cinder.tests.unit.api.contrib.test_volume_host_attribute
- cinder.tests.unit.api.contrib.test_volume_manage
- cinder.tests.unit.api.contrib.test_volume_migration_status_attribute
- cinder.tests.unit.api.contrib.test_volume_tenant_attribute
- cinder.tests.unit.api.contrib.test_volume_unmanage
- cinder.tests.unit.api.v2.test_volumes

Most changes in this patch:
- make sure that Request.body is set to bytes;
- replace jsonutils.dumps with jsonutils.dump_as_bytes;
- replace json.loads with oslo_serialization.jsonutils.loads;
- replace dict.iteritems with dict.items.

Partial-Implements: blueprint cinder-python3
Change-Id: Icbb96ff84b7012b58f7296eea4fbcd620e081614