tls: allow obvious key/passphrase combinations

Passphrase is now used whether keys are provided singly, in an array of
string/buffer, or an array of object, where it used to be ignored in
some argument combinations. Specifically, these now work as expected:

  key: [encryptedPem],
  passphrase: 'passphrase'

and

  key: [{pem: encryptedPem}]
  passphrase: 'passphrase'

and

  key: [{pem: unencryptedPem}]

PR-URL: https://github.com/nodejs/node/pull/10294


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>