default path in /tmp should include username

The default directory of /tmp/flume is messy on a multiuser system. It also causes problems if a user follows the flume tutorial using packages, since the package-installed flume node runs as "flume" whereas they probably tried "flume dump" as a regular user, causing /tmp/flume to become owned by them.

We can set the default to /tmp/flume-${user.name} to avoid this.