rng_dev_path in nova.conf template configurable in node attribute

added attribute to make rng_dev_path in nova.conf configurable with the
node attribute default['openstack']['compute']['libvirt']['rng_dev_path']. Default is set to nil to let this parameter stay an optional one.

Change-Id: I1bcf67eec0b8918a3ba644d76cafb20ab89ce704
Merge "rng_dev_path in nova.conf template configurable in node attribute"