config: Expose RNG related config

This patch exposes two config options:

1, VirtIoRngDeviceSupported: Defines whether VirtIO Random Number
Generator devices is supported for given cluster level,

2, ClusterRequiredRngSourcesDefault: For UI only. Random Number
Generator sources that are preselected when creating a new cluster
(comma separated string of values 'random' and 'hwrng').

Change-Id: I2938e5e4b05093b4b9465929f29b8928022dd613
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/977079