auth_strategy has not longer to be set to keystone

After merge of Ife63dc7ab680fcf0588d95c98fc9f29901485c64 it's
not longer necessary to set auth_strategy = 'keystone', because
auth_strategy is now keystone by default.

Change-Id: I04d27cfaf1ff60be89ba76301da1d1407a158145
Closes-Bug: #1312932
Merge "auth_strategy has not longer to be set to keystone"