container: change XxxContainers() return behaviour

Problems: previously when a NewContainer() creationg fails the whole
process would return a nil. That would be not acceptable for large
number of containers. I.e: if we had 100 containers, one buggy
container retrieval would cause to net get the rest 99 containers.

Solution: return all successfull retrieved containers and silently fail for
failing containers.
Merge pull request #4 from fatih/master.containers

container: change XxxContainers() return behaviour