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>
Are you suggesting map from name to path? Does that stop us from evolving the Namespace struct in future?