`RocketMQProperties#nameServer` should offer a more user-friendly format

The `nameServer` property ultimately maps to `ClientConfig#setNamesrvAddr` that requires a `host:port` pair separated by `;`

I think it would be nicer to not expose that complexity back to the configuration. Rather, the configuration should expose a `List` where each item represent a `host:port` pair.

There are several advantages to this:

* Each pair can be checked individually and an exception can be thrown if its format is invalid
* YAML configuration is much more readable as you can use the native list format
* Perhaps the property can be renamed to `servers` to express it is a list

Taking this into consideration (+ #9) the property can be configured as follows:

```yml
rocketmq:
servers:
- 10.0.0.1:9876
- 10.0.0.2:9876
```

Start with RocketMQProperties#nameServer and trace how it maps to ClientConfig#setNamesrvAddr. Check how YAML configuration is bound and where validation belongs; done means accepting a list of host:port values, validating each item, and producing the proposed servers configuration without exposing the semicolon-separated format.

Written by the indexing model from the issue text.