Zookeeper authentication related properties to support CredentialProviders

The hadoop.zk.auth and ha.zookeeper.auth properties currently support either a plain-text authentication info (in scheme:value format), or a @/path/to/file notation which points to a plain-text file.

This ticket proposes that the hadoop.zk.auth and ha.zookeeper.auth properties can be retrieved via the CredentialProviderAPI that's been configured using the credential.provider.path, with fallback provided to the clear-text value or @/path/to/file notation.