Retain repository archives across executions

Created by: mrnugget

Copying from the CHANGELOG:

• The default behaviour of src campaigns [preview|apply] has been changed to retain downloaded archives of repositories for better performance across re-runs of the command.
• To use the old behaviour and delete the archives use the -clean-archives flag.
• Repository archives are also not stored in the directory for temp data (see -tmp flag) anymore but in the cache directory, which can be configured with the -cache flag. To manually delete archives between runs, delete the *.zip files in the -cache directory (see src campaigns -help for its default location)

I'm still not sure whether this is the best way to achieve the goal of "do not delete the same zip file over and over again", because it's easy ot have the cache fill up: if you run campaigns multiple times and the base ref of the repository changes you get a new zip every time.

While I keep thinking about this and hacking on it: opinions?