Allow global image/delta max download attempt and interval variables

[CameronDiver]

Depends on: https://github.com/balena-io/balena-supervisor/issues/1064

We should allow users to specify the maximum amount of times that the supervisor should attempt to download an image or a delta, and the interval between the download attempts. This is implemented already for delta downloads, but this should be allowed globally (with the delta vars overriding).

If we reach the max amount of download attempts, with no successful download, we should take the download lock (requiring the implementation of https://github.com/balena-io/balena-supervisor/issues/1064) to avoid maxing out metered connections. This behaviour may also be better to be behind a configuration variable (some users may want to fall back to a full image download when the delta doesn't work for whatever reason).