Add option parsing to nsenter and enable specifying commands with arguments

1. Adds option parsing.
2. Adds a flag to nsenter as root with full capabilities. This allows admins to modify settings in the container namespaces.
3. As a side effect of using getopt option parsing, commands can be specified with arguments by using -- cmd --arg1 <val1> --arg2 <val2> --arg3

Docker-DCO-1.1-Signed-off-by: Mrunal Patel <mrunalp@gmail.com> (github: mrunalp)
No, they are required and available as context.String("containerjson") and so on.