add spec for exec a new process inside a container

This attempts to create a spec for execin process behavior for v1 container specification

Relevant to #286 

@crosbymichael @vishh @rjnagal feel free to take this over and change it if i'm missing somethings.
How about - "When the process is stopped, all its children will also be stopped and the sub-cgroups tracking them will also be removed."