Use a secure random generator to seed nonce for digest authentication

Use a secure random generator to seed nonce for digest authentication

https://tools.ietf.org/html/rfc7616 says:

5.12.  Parameter Randomness

The security of this protocol is critically dependent on the
randomness of the randomly chosen parameters, such as client and
server nonces.  These should be generated by a strong random or
properly seeded pseudorandom source (see [RFC4086]).

Change-Id: I4da5316cb1eb3f59ae06c070ce1c3335e9ee87d6
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>