Make RSA key size configurable

[slintes]

With this change the RSA key size is made configurable.

The given keysize is checked to be a number and to be >= 512... not sure if there should be more checks?

Fixes #111

[slintes]

just rebased on master

[simonswine]

Thanks for that, not too sure how non well known rsa key sizes (!= 1024, 2048, 4096) are handled by let's encrypt, but this shouldn't break anything