adding auth client isEnabled check before loading the properties file fo...

...r authentication client configuration

based of , https://github.com/caskdata/cdap-ingest/pull/72, opening against release/1.0.0 as per @poornachandra suggestion.
Should just do the close using finally in here. It makes the code much simplier.

```java
InputStream is = new FileInputStream(...);
try {
  properties.load(is);
} finally {
  Closeables.closeQuietly(is);
}