FIX: support for private key protocol

[dommi22m]

For now there is no support to choose the protocol for private key. (3DES/AES) Neither for the auth key (MD5/SHA)

So you will get an TypeError if not AES is selected in Synology SNMP Setup :

float() argument must be a string or a number, not 'NoneType'

I made a little change to get the private key protocol as parameter. Not well programmed but yet an acceptable change.