Allow public_key:pem_entry_decode/2) to handle AES-128-CBC ciphered keys

Private keys generated by modern versions of ssh-keygen are ciphered
with AES-128-CBC instead of DES-EDE3-CBC.


Since DES-EDE3-CBC ciphered keys are handled, and the underlying
support for AES-128-CBC is already present, it seems a bug of omission
that AES-128-CBC ciphered keys are not.

In case it's not obvious, this is so the ssh application can use the user's 
keys in ~/.ssh