Use method key type instead of EVP_PKEY_RSA

Make RSA method more flexible by using the key type from the
method instead of hard coding EVP_PKEY_RSA: by doing this the
same code supports both RSA and RSA-PSS.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2177)