Bump oslo.utils version to 1.6.0
Bump oslo.utils version to 1.6.0

Sync with global requirements.

Change-Id: Id65c14b65a192ef76d7f5c8bee247c7d9ec581a8

stackforge/python-senlinclient,openstack/python-senlinclient,stackforge/python-senlinclient,openstack/python-senlinclient