Update compute driver name for Ironic

Commit 08448e3a mistakenly updated the location of the Ironic compute
driver to be loaded from within the Nova tree.  However, no such module
exists as the compute and scheduler drivers both live in the Ironic tree.
This reverts that and adds a note explaining the setting.

Change-Id: I7f32f77bd7e5fd1f6b41014ac71bbf6e1a7d84cf
Merge commit 'refs/changes/49/81849/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch