Update compute_driver in documentation
Update compute_driver in documentation

Nova doesn't support long driver names for compute_driver.
Replace long driver name 'nova.virt.ironic.IronicDriver' with short one
'ironic.IronicDriver'.

Reference:
https://review.openstack.org/309504/

Change-Id: I00713abd3c230a42380593ff39c238b448bc0732

SauloAislan/ironic,pshchelo/ironic,SauloAislan/ironic,bacaldwell/ironic,NaohiroTamura/ironic,openstack/ironic,devananda/ironic,bacaldwell/ironic,pshchelo/ironic,openstack/ironic,NaohiroTamura/ironic