Check if auth is enabled before trying to load auth client properties.

Simply makes the following actions conditional upon `isAuthEnabled`:
-reading of auth client properties file
-configuring auth client

Solves the same problem as: https://github.com/caskdata/cdap-ingest/pull/72, but in file-tailer. (https://github.com/caskdata/cdap-ingest/pull/72 resolves the issue in flume).

Just as reference, [here](https://s3.amazonaws.com/uploads.hipchat.com/26476/1011205/PVXeULnifjRyKby/Screen%20Shot%202014-10-09%20at%2011.17.07%20PM.png) is a screenshot of logs, when omitting authclient.properties file, even if authentication is disabled in cdap, which this fix resolves.
i know this is not part of your PR. , can you add the exception e to the log message.