Change httpd Keystone access log to keystone_access.log

Keystone's access log was going to httpd/access.log, which is the
common place for all access logging. This made it difficult to see
Keystone accesses apart from other access. Keystone's access log
will now be keystone_access.log

This makes the Keystone configuration similar to Horizon which uses
horizon_access.log.

Change-Id: I6e5ac121302b3d138758e6c49dffa9f05ad2fb85
Partial-Bug: #1359995
Merge "Change httpd Keystone access log to keystone_access.log"