Refactor Druid HTTP client to support extending a base Druid client

[jtuglu-netflix]

Overview

This allows for extensibility of Druid Connector client (through inheritance). Implementing a TLS-supported (or whatever signing mechanism you have) client is now much easier as users can apply the necessary overrides of the RestTemplate after Base ctor returns.

Changes

• Change Coordinator URI to Router URI, as this is how Druid clusters are accessed now.
• Create BaseDruidHttpClientConfig which is an abstract class that specifies the Bean(s) to implement for any MetcatDruidClient.
• Modify DruidConnectorFactory to take in any class which extends the BaseDruidHttpClientConfig class.

Notes:

• Idea here is that to extend the default Druid connector, you can create your own *Plugin.java file, and pass the custom MetacatDruidClient client implementation class type into the DruidConnectorFactory ctor.
• Having an abstract class instead of a BaseDruidHttpClientConfig with default impls is regrettable, but I was getting duplicate Bean name warnings when instantiating multiple instances of the interface.