ssl: set engine implicitly when a PKCS#11 URI is provided

This allows the use of PKCS#11 URI for certificates and keys without
setting the corresponding type as "ENG" and the engine as "pkcs11"
explicitly. If a PKCS#11 URI is provided for certificate, key,
proxy_certificate or proxy_key, the corresponding type is set as "ENG"
if not provided and the engine is set to "pkcs11" if not provided.

Acked-by: Nikos Mavrogiannopoulos
Closes #2333