OOD proxying assumes only single login node

[sjpb]

OOD defaults are potentially broken for >1 login node.

1. Everything layout defines openondemand as all login, so could have multiple openondemand nodes

environments/common/layouts/everything:
[openondemand:children]
# Host to run Open Ondemand server on - subset of login
login

1. Clearly grafana can only proxy a single OOD node, so this proxy config assumes openondemand_servername is the same on all hosts:

environments/common/inventory/group_vars/all/grafana.yml:grafana_url_openondemand_proxy: "https://{{ openondemand_servername | default('') }}/node/{{ groups['grafana'].0 }}/{{ grafana_port }}"

1. The arcus definition is actually ok for this:

environments/arcus/inventory/group_vars/all/openondemand.yml:openondemand_servername: "{{ hostvars[ groups['openondemand'] | first].ansible_host }}" # Use a SOCKS proxy to acccess

but this will be really easy to mess up

[sjpb]

Tried to fix this but you can't do:

grafana_url: "https://{{ hostvars[groups['openondemand'].0]['openondemand_servername'] }}..."

when openondemand_servername is also accessing hostvars - the "outer" use of it does not get a templated variables. Which kind-of makes sense.