update default -rpcsslciphers to include TLSv1.2

- this extends the accepted ciphersuites with TLSv1.2 ones
- also removes !AH, as I could not find documentation on it and the change
  did not result in a changed ciphersuite list (checked via openssl
  ciphers -v)
- closes #3096 (which also contains more details)
Merge pull request #3104 from Diapolo/rpcssl-defaultciphers

update default -rpcsslciphers to include TLSv1.2