Lava doesn't clear some tmp data

Issue Background

/var/lib/lava/dispatcher gradually increases in size - my vm had 0.5GB there

Issue description

Files are downloaded to /var/lib/lava/dispatcher/slave/tmp for each job but are never cleared, either there needs to be a lava update or we're going to need to clear them manually

Related with issue #82 (closed)

Acceptance criteria

When the disk space in /var/lib/lava/dispatcher/ stops ballooning