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
Merge pull request #7299 from Pewpewarrows/patch-1

Change hardcoded padding numbers to M2Crypto's exposed constant