Updated tinyDTLS test apps to use identity hint

Updated tinyDTLS test apps to use identity hint

dtls-server test is updated to pass 'psk hint' to tinyDTLS library so that
it can send ServerKeyExchange packet. Similarly, dtls-client test
is updated to retrieve PSK specific for the server whose identity was
received via 'psk hint' packet.

Change-Id: Ifba720f1505ed6afbf56dfc179d98790176ce0b0
Signed-off-by: Sachin Agrawal <sachin.agrawal@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/348
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Seung-Woo Lee <sw0524.lee@samsung.com>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>