Don't open PSM context when run on single node

When running many ranks (err, processes) on a single node using PSM, it's possible to
exhaust the network hardware contexts (there are 16).  This patch checks
if only a single node is being used. If so, the 'ipath' component of PSM
is disabled and no hardware contexts are opened.
(cherry picked from commit 75f66605e568b87b4ca988ed1bfa939cd1574773)