Provide usable defaults for Open Ondemand

[sjpb]

Updates the common environment to provide a default set of Open Ondemand variables based on the caas and stackhpc environment configurations. This means no configuration of Open Ondemand is necessarily required for a new environment.

Fixes #210

[sjpb]

Tested on Azimuth (to ensure all proxying etc still works), all ok:
hpctests: ok
shell: ok
jupyter: ok
desktop: ok
monitoring: ok
active jobs: ok