Rename param to defer-days, add reporting for deferred containers, general simplifications

[pjdubya]

This update makes the following changes:

• Renamed parameter from delay-days to defer-days for better clarity in reporting
• Added reporting on number of containers that were deferred to eliminate any confusion about why items in Scanned count weren't in Updated count
• Simplified login in update process for readability and maintainability
• Fix problem where deferred items were reported as updated when they weren't
• Added unit test cases to test defer functionality

These changes are in anticipation of containrrr#1895 in the main repo and are aligned with that new logic.

[pjdubya]

In case anyone else needs a version of watchtower that supports the new defer-days parameter prior to the main containerr/watchtower merging in my PR over there, I published a docker image to Docker Hub for now that provides that support. Retrieve it with

docker pull pjdubya/watchtower:latest

Run it the same way as you ran watchtower in the past, but add the following if you want to enable that feature, replacing 1 with however many days you want to wait before installing newly published images.

--defer-days 1

As soon as the PRs are merged into containerr/watchtower I'll delete the docker image above, so it should only be a temporary workaround for folks.