OMERO.web app configuration style unification and fixes

[sbesson]

While reviewed the configuration changes in https://github.com/ome/prod-playbooks/pull/332, we realised that our Ansible playbooks use very different styles for setting the OMERO.web app configuration:

• sls-gallery and learning explicitly set the OMERO.web configuration using the primary omero_web_config_set - see https://github.com/ome/prod-playbooks/blob/d21c6f772469cbcbbbe9ff8dfabce350ed38f863/sls-gallery.yml#L81-L111 and https://github.com/ome/prod-playbooks/blob/d21c6f772469cbcbbbe9ff8dfabce350ed38f863/learning.yml#L82-L118
• ome-demoserver.yml converts three Jinja template containing a series of config append and config set subcommands into configuration files https://github.com/ome/prod-playbooks/blob/d21c6f772469cbcbbbe9ff8dfabce350ed38f863/ome-demoserver.yml#L213-L246
• nightshade-webclients.yml uses a combination of omero_web_apps_names, omero_web_apps_packages, omero_web_apps_top_links, omero_web_apps_config_append and omero_web_apps_config_set - see https://github.com/ome/prod-playbooks/blob/d21c6f772469cbcbbbe9ff8dfabce350ed38f863/nightshade-webclients.yml#L113-L169
• omero/training-server/playbook.yml copies a .omero file containing a series of omero config set and omero config append commands into the configuration directory - see https://github.com/ome/prod-playbooks/blob/d21c6f772469cbcbbbe9ff8dfabce350ed38f863/omero/training-server/playbook.yml#L182-L191

For comparison, the IDR deployment playbooks use a style most closest to nightshade-webclients i.e. the built-in omero_web_apps* variables supplemented by omero_web_apps_config_append and omero_web_apps_config_set -see https://github.com/IDR/deployment/blob/77b759ce21f2a165903aa5cbea7fcb673fcf55a8/ansible/group_vars/omero-hosts.yml#L162-L604

The mixture of styles is particularly confusing. Deciding on the preferred style(s) and applying them systematically would certainly reduce maintenance.

A related issue highlighted by the progression https://github.com/ome/prod-playbooks/pull/332 is that extreme care should be put in the usage of the _append variables. The configuration files stored under /opt/omero/web/config are loaded as part of the omero-web service lifecycle. Internally omero config append includes some logic avoiding the duplication of a value in a configuration list. However, removing a value from a list e.g. unregistering a web app requires more thoughts