core: AsyncTaskType enum - added downloadImage

Added downloadImage to AsyncTaskType enum in order to construct
CommandAsyncTask instead of SPMAsyncTask (determined at
AsyncTaskFactory -> construct method).

Change-Id: I2c999b5b4426cf9b58691f2a1ee1c247a152ed7c
Bug-Url: https://bugzilla.redhat.com/1074311
Signed-off-by: Daniel Erez <derez@redhat.com>