Added the ability to easily change the work factor in configurable hashing algorithms

This addition allows users to easily change the 'work factor' for bcrypt or 'iterations' for pbkdf2 by setting BCRYPT_ROUNDS and PBKDF2_ITERATIONS, respectively without having to create their own hasher. 

There are checks in place to update the password with the new work factor as required.

Ticket: https://code.djangoproject.com/ticket/19043