core: Add audit log for export repo image

Adding an audit log when exporting a repository image (same as
importRepoImage)

Bug-Url: https://bugzilla.redhat.com/1021210
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Change-Id: Ifb79102b01b596a1ff74a1a38fdcb7dca71e0c7c