virtng: add random device by default

* Salt minion is unable unencrypt the messages from master during boot
    because of lack of entropy, throwing the exception:

    File "/usr/lib/python2.7/dist-packages/salt/utils/rsax931.py", line 146, in sign
    raise SSLError('Unable to encrypt message')
    SSLError: Unable to encrypt message:
    error:80064191:lib(128):osrandom_init:getrandom() initialization failed with EAGAIN. Most likely Kernel CPRNG is not se
    error:80065190:lib(128):osrandom_rand_bytes:getrandom() initialization failed.
    error:04088003:rsa routines:RSA_setup_blinding:BN lib
    error:04066044:rsa routines:RSA_EAY_PRIVATE_ENCRYPT:internal error

    After node has been booted up, and /dev/random device collected some
    extra entropy, salt-minion could start.

    This patch configures libvirt vms to use /dev/urandom for faster
    entropy regeneration

Change-Id: I470166b4424752d24ac4bb2cb87d9f99cd14752e
Co-Authored-By: Oleksandr Savatieiev <osavatieiev@mirantis.com>
Prod-Related: PROD-19711

_modules/virtng.py

          start=True,  # pylint: disable=redefined-outer-name
          disk='default',
          saltenv='base',
-         rng={},
+         rng=None,
          **kwargs):
     '''
     Initialize a new vm
         salt 'hypervisor' virt.init vm_name 4 512 nic=profile disk=profile
     '''
 
+    rng = rng or {'backend':'/dev/urandom'}
     hypervisor = __salt__['config.get']('libvirt:hypervisor', hypervisor)
 
     nicp = _nic_profile(nic, hypervisor, **kwargs)