Redact x-subject-token from response headers

When you invoke any OpenStack API of any of the OpenStack services
e.g. glance, neutron, cinder, heat, ceilometer, nova, keystone
then it logs readable x-subject-token at the debug log level in the
respective log files.

Simply redacting the x-subject-token in keystone client response header
before logging it.

SecurityImpact
Closes-Bug: #1371355
Change-Id: Iac16c6358250677544761beea9f5c5d8ba29afac
Merge "Redact x-subject-token from response headers"
Test commit