ZFSSA volume driver REST client python3 fixes
ZFSSA volume driver REST client python3 fixes

1) Although it's unlikely that non-ASCII characters would ever appear in
API requests, encode/decode("utf-8") handles bytes vs. strings.

2) In python2, HTTPMessage.getheader() was equivalent to
HTTPMessage.get(), but only the latter is available in python3.

Change-Id: I3fce732e46ae3d16d753af053883520ee7b1f10a
Closes-Bug: #1792839

mahak/cinder,openstack/cinder,openstack/cinder,mahak/cinder