Revert "Remove auth_strategy from nova.conf"

This reverts commit a3fdcba84f6f3d6e47ee112b934a839773f27582.

While trunk Nova has indeed made the referenced changed to default to
keystone, Icehouse has not, and we want to deploy current stable +
trunk from TripleO trunk.

Closes-Bug: #1317854
Change-Id: I42acc3851811703960feb40bc4ce0e895ee57c05
Merge "Revert "Remove auth_strategy from nova.conf""