Install openssh so jupyter-sshd-proxy could work

Follow-up to #585 (comment).

The conda-forge package doesn't actually pull in openssh,
which feels like the right call for now (as systems may already
have openssh via other means?). 

Explicilty add openssh so jupyter-sshd-proxy works

pangeo-notebook/environment.yml

@@ -79,6 +79,7 @@ dependencies:
  - numpy
  - numpy_groupies
  - odc-stac
+ - openssh
  - pandas
  - panel
  - parcels