core: introducing ImageSpmCommand

This patch extracts ImageSpmCommand as a parent class of UploadStreamCommand to
contain shared logic between this command and the to-be-added
RetrieveImageDataCommand.

Change-Id: I32166ee993bce33680f6b2feb5359666e414c777
Bug-Url: https://bugzilla.redhat.com/1136840
Signed-off-by: Liron Aravot <laravot@redhat.com>