doc: Fixes parameter name in rbd configuration on openstack havana/icehouse

doc: Fixes parameter name in rbd configuration on openstack havana/icehouse

The documentation for using rbd together with openstack havana/icehouse
states that the parameter libvirt_disk_cachemodes should be added to
the nova.conf file. However, this is the only parameter that has no
legacy name with a 'libvirt_' prefix. (See
https://github.com/openstack/nova/blob/icehouse-eol/nova/virt/libvirt/driver.py#L252
for the configuration option)
Thus the configured disk_cachemodes were not applied, defaulting to
no caching.

Fixes: #17978
Signed-off-by: Michael Eischer <michael.eischer@fau.de>