Add curator based ZooKeeper communication support over SSL/TLS into the common library

With HADOOP-16579 the ZooKeeper client is capable of securing communication with SSL. 

To follow the convention introduced in HADOOP-14741, proposing to add to the core-default.xml the following configurations, as the groundwork for the components to enable encrypted communication between the individual components and ZooKeeper:

• hadoop.zk.ssl.keystore.location
• hadoop.zk.ssl.keystore.password
• hadoop.zk.ssl.truststore.location
• hadoop.zk.ssl.truststore.password

These parameters along with the component-specific ssl.client.enable option (e.g. yarn.zookeeper.ssl.client.enable) should be passed to the ZKCuratorManager to build the CuratorFramework. The ZKCuratorManager needs a new overloaded start() method to build the encrypted communication.

• The secured ZK Client uses Netty, hence the dependency is included in the pom.xml. Added netty-handler and netty-transport-native-epoll dependency to the pom.xml based on ZOOKEEPER-3494 - "No need to depend on netty-all (SSL)".
• The change was exclusively tested with the unit test, which is a kind of integration test, as a ZK Server was brought up and the communication tested between the client and the server.
• This code change is in the common code base and there is no component calling it yet. Once YARN-11468 - "Zookeeper SSL/TLS support" is implemented, we can test it in a real cluster environment.