Change namespaces config to include path for setns

This changes the namespace configuration on the config to include the
name of the namespace along with an optional path.  This path is used to
point to a file of another namespace for the namespace so that it can be
joined in place of the empty, initialized namespace.

Closes #283

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
libcontainer is a low level API so i don't think this is a big issue for people, it maybe a little harder but I would rather have a flexible and consistent way to enforce the changes then the `map[string]string` where you don't know what is supposed to be in the key and why some have an empty string as the value.