Change hardcoded padding numbers to M2Crypto's exposed constant

As defined here:

https://github.com/eventbrite/m2crypto/blob/master/SWIG/_rsa.i#L27

https://github.com/openssl/openssl/blob/master/crypto/rsa/rsa.h#L297

In addition this line should use an explicit constant as well, but I wasn't able to track down what it stands for:

https://github.com/saltstack/salt/blob/develop/salt/crypt.py#L214