Add option to send mails without authentication

The default behaviour of Azkaban is to send emails with mail.smtp.auth set to true. However, our internal SMTP server supports sending of emails without authentication for certain accounts, using the default Azkaban behaviour would throw the following error.

```
[Emailer] [Azkaban] Email message send failed
javax.mail.AuthenticationFailedException: No authentication mechansims supported by both server and client
        at com.sun.mail.smtp.SMTPTransport.authenticate(SMTPTransport.java:760)
        at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:673)
        at javax.mail.Service.connect(Service.java:295)
        at javax.mail.Service.connect(Service.java:176)
        at azkaban.utils.EmailMessage.sendEmail(EmailMessage.java:197)
        at azkaban.utils.Emailer.sendErrorEmail(Emailer.java:153)
        at azkaban.utils.Emailer.alertOnError(Emailer.java:203)
        at azkaban.executor.ExecutorManager.finalizeFlows(ExecutorManager.java:986)
        at azkaban.executor.ExecutorManager.access$1100(ExecutorManager.java:60)
        at azkaban.executor.ExecutorManager$ExecutingManagerUpdaterThread.run(ExecutorManager.java:910)
```

This commit allows emails to be sent without any authentication. Adds an optional boolean parameter mail.useAuth (defaults to true) in azkaban.properties.

Yes, there shouldn't be a need to. I was kinda puzzled as well as to why Emailer attributes didn't make use of the public methods in the AbstractMailer... didn't put too much thought into it and just followed the existing code to make it consistent. 

If you'd like I could modify Emailer to have it grab the useAuth property from AbstractMailer instead.