Typo in Security Guide Neutron Flow image

There was a typo in the Neutron flow image in the
security guide document. Lists one of the flows as
a "Tentant API" rather than "Tenant API". I modified
the png image and pushed as part of this commit.

http://docs.openstack.org/security-guide/content/ch033_securing-neutron-services.html

Change-Id: I1ec822972f25fd62cc9a5077975f7b556db10664
backport: havana
Closes-Bug: 1253832
Merge "Typo in Security Guide Neutron Flow image"