Container management functionality

[mslacken]

Add a more advanved container management functionality to warewulf.
close #1067

[mslacken]

@anderbubble @griznog This PR will add a

wwctl container reimport

command which will recreate a container from its original source. I think this useful, when you have messed your container installation process and want a fresh start. What do you think?
I implemented this more by accident as the cache is already in warewulf, so a reimport just pulls the old references.

[anderbubble]

I like the idea in theory; but how does it know what container to reimport from? What happens if the cache is cleared?

[mslacken]

I like the idea in theory; but how does it know what container to reimport from?

At import time additional information is written to a meta file, so that the metainformation of the container is preserved

What happens if the cache is cleared?

An error! As a cache clearing mechanism isn't implemented, this is not really a problem.

My idea is, to also add the possibility to add new layers to cache, if a container is modified. This would make it possible to push/export containers back to a registry.