Currently nova.conf.j2 generates two compute_driver options for
nova_compute_ironic container like this:
compute_driver = ironic.IronicDriver
compute_driver = libvirt.LibvirtDriver
nova_compute_ironic container fails to start because the latter value
overrides the former one.
This patch fixes the issue recently introduced in [1].
[1] 63314ad6dd181a0e975438de2e99409238f1b775
Change-Id: Ibb661a5a594120be4195d331c38883c3b2886361
Closes-Bug: #1706534
c1ed80fe84