Slice expiry reminder emails and redirect nginx logs to files for file beats

[kthare10]

• Added support in audit to check for slices close to expiry and sends warning emails to the slice owners if the slice is set to expire in less than 12 hours or 6 hours.
• Uses mail.fabric-testbed.net
• Also updated nginx config to redirect access and error logs to a location to be picked up by filebeats