app-container: try out "root-container" naming

Based on feeback from a number of folks the overloading of the
"container" has caused quite a bit of confusion. This PR tries
"root-container" on for size. Which if you think of cgroups as heirarchy like
the filesystem (which they are) makes a lot of s

This is an alternative to using a new term such as "pod", "grou
"cell". The idea is that we don't call `/` on the UNIX filesyst anything
special, we call it the "root-directory". So, lets see can try this on for
size here. There is a root-container and on sub-containers. Apps execute as a
direct sub-container of the root-container.

https://github.com/coreos/rocket/issues/151