Change httpd Keystone log to keystone.log

The normal extension for log files is .log. Log rotation should
work because the extension is as expected.

Change-Id: Ia5e42ea9d953e8395b86ab58cdef6c2d852efc94
Related-Bug: #1359995
Merge "Change httpd Keystone log to keystone.log"
Merge branch 'master' of https://github.com/openstack-dev/devstack