Add DevNonce Clear Functionality (#49)

Details:
- DevNonce clear functionality helps to remove older device
activation records from device_activation table in NS.
- Using this method we can clear older or already generated device activation
records from database to prevent "DevNonce already exits" error in OTAA method

*Note: Server keeps latest 20 records in device_activation table of NS database
for manage recent device activation.

Signed-off-by: SAGAR PATEL <sagar.a.patel@slscorp.com>