[Cinder] Can't upload volumes with metadata to images

Release: MOS 7

Steps to reproduce:
1) Create a volume from an image
2) Make sure that the volume has non-empty description and os_type in the metadata
(here is a sample of such volume - http://paste.openstack.org/show/He0BeLUVU16Wv4y1Gpto/)
3) Try to upload the volume to Glance as an image

Actual result:
The uploading fails. Here is the corresponding message in the log file:
2017-10-12 12:37:10.325 27149 INFO cinder.api.openstack.wsgi [req-86896d07-d877-411c-a67b-b3d8f6e99cc2 8ae2b1945c3f99c846702371bf73def2bdc050778dc4bace18631d526cfeb3da 3867ca17e842482bab91fb52a183d9ec - - -] HTTP exception thrown: Unable to set 'properties' to '{u'os_type': u'windows', u'description': u''}'. Reason: {u'os_type': u'windows', u'description': u''} is not of type u'string'
{u'description': u'', u'os_type': u'windows'}

Expected result:
The volume has been successfully converted to image and uploaded to Glance

Notes:
It looks like the following upstream bug - https://bugs.launchpad.net/cinder/+bug/1527324