To bring this in line with new changes on master, moving objects from
lava_dispatcher/tests to tests/lava_dispatcher. Tidying up affected
import lines.

Also tidying how DownloadAction is instantiated from Musca deploy.

Change-Id: Ibf3b3d5df4eb3038f7e71ce6eb1d495d327beb80
Signed-off-by: Dean Birch <dean.birch@arm.com>