Specifying a Python list for config `bootstrap.servers` fails silently with no errors or indications

Description
===========
The configuration key `bootstrap.servers` implies that multiple servers can be specified, but it is not explicitly defined what this key's value should be. It appears that only a *string* using comma separation is allowed for this config key. If a Python list is given, no explicit error or feedback is given to indicate this is not valid. Instead, calls to consumer.poll() or other broker-bound functions simply block and never succeed.

Ideally, a Python list (e.g. `["broker1", "broker2"]`) should be valid; or at the very least, constructing the Consumer should throw a runtime/configuration exception.

How to reproduce
================
Simply specify a Python list as the value for `bootstrap.servers` in the config given to the Consumer.

```python
conf = {
'bootstrap.servers': ['kafkabroker1:9092', 'kafkabroker2:9092'],
...
}
c = Consumer(conf)
while True:
c.poll(1) # this will never produce a message due to the Python list specified above as the bootstrap.servers value
```

Checklist
=========
Please provide the following information:

- [x] confluent-kafka-python and librdkafka version (`confluent_kafka.version()` and `confluent_kafka.libversion()`):
```
>>> confluent_kafka.version()
('1.2.0', 16908288)
>>> confluent_kafka.libversion()
('1.2.0', 16908543)
```
- [ ] Apache Kafka broker version:
- [ ] Client configuration: `{...}`
- [x] Operating system: Ubuntu 18.04 and CentOS 7
- [ ] Provide client logs (with `'debug': '..'` as necessary)
- [ ] Provide broker log excerpts
- [ ] Critical issue