Add support for wolfSSL Context password callback

[tmael]

This PR adds support to register a default password callback.

The load_cert_chain() python API can now accept the password parameter encoded using UTF-8.

You can use the set_passwd_cb() python API to register a callback function that sets the password before loading an encrypted key using a call to load_cert_chain().

The builder script has been updated to include wolfSSL_CTX_set_default_passwd_cb() wolfSSL API.

This password callback feature allows urllib3 tests in wolfSSL/osp#36 to work:

TestHTTPS::test_client_key_password
TestHTTPS_TLSv1::test_client_key_password
TestClientCerts::test_client_cert_with_string_password
TestClientCerts::test_client_cert_with_bytes_password
TestClientCerts::test_load_keyfile_with_invalid_password

Here's an example of how to use it:

    def load_cert_chain(self, certfile, keyfile=None, password=None):
        if password is not None:
            if not isinstance(password, six.binary_type):
                password = password.encode("utf-8")
            self._ctx.set_passwd_cb(lambda *_: password)
        self._ctx.load_cert_chain(certfile, keyfile=keyfile, password=password)