Update nova.conf.sample for python-keystoneclient 0.5.0

python-keystoneclient 0.5.0 added a new oslo.config option:

 enforce_token_bind=permissive

Change-Id: If719600866561e03b037b1eb1f58e14103a99385
Merge "Update nova.conf.sample for python-keystoneclient 0.5.0"