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.
This one line (56) seems to already be documented at line 31 of this file.
The verifySSLCert doc addition (line 57) is good though.