Update some options in nova.conf.erb

Update option compute_scheduler_driver to scheduler_driver.
Remove option max_cores since it is not appeared in latest release.

Reference below patch for details
* https://review.openstack.org/#/c/26625/
* https://review.openstack.org/#/c/15068/

Change-Id: I7f98af8225fdcd6864cec21cf1b1fc0cc075356f
Closes-Bug: #1389624