core: Added download_image to StepEnum for monitoring

Added DOWNLOAD_IMAGE to StepEnum and ExecutionMessages
for proper task monitoring (currently used by
ImportRepoImageCopyTaskHandler and DownloadImageVDSCommand).

Change-Id: I2ee2669edff2d385d759fba3bf7caf77d89efecf
Related-To: https://bugzilla.redhat.com/1074311
Signed-off-by: Daniel Erez <derez@redhat.com>