Handle pulp_settings merging better

[rooftopcellist]

Is your feature request related to a problem? Please describe.

Currently, there is a potential for the tasks that combine pulp_settings to fail if the common role has not completed. This is because in pulp.yml, we reference pg variables that are not set until the postgres_configuration_secret is read in. This results in an error like this:

TASK [pulp-web : Combining pulp_settings] **************************************\r\n\u001b[1;30mtask path: /opt/ansible/roles/pulp-web/tasks/main.yml:9\u001b[0m\n\u001b[0;31mfatal: [localhost]: FAILED! => {\"msg\": \"The task includes an option with an undefined variable. The error was: {'api_root': '/api/galaxy/pulp/', 'cache_enabled': True, 'db_encryption_key': '/etc/pulp/keys/database_fields.symmetric.key', 'galaxy_collection_signing_service': 'ansible-default', 'galaxy_container_signing_service': 'container-default', 'ansible_certs_dir': '/etc/pulp/keys/', 'databases': {'default': {'HOST': '{{ postgres_host }}', 'ENGINE': 'django.db.backends.postgresql_psycopg2', 'NAME': '{{ postgres_database }}', 'USER': '{{ postgres_user }}', 'PASSWORD': '{{ postgres_pass }}', 'PORT': '{{ postgres_port }}', 'CONN_MAX_AGE': 0, 'OPTIONS': {'sslmode': '{{ postgres_sslmode }}'}}}, 'static_root': '/app/galaxy_ng/app/static/', 'redis_host': '{{ ansible_operator_meta.name }}-redis-svc', 'redis_port': 6379, 'redis_password': ''}: 'postgres_host' is undefined

Describe the solution you'd like

We should combine these variables after the postgres configuration secret is set inside the common role rather than templating them like this:

      databases:
        default:
          HOST: "{{ postgres_host }}"
          ENGINE: django.db.backends.postgresql_psycopg2
          NAME: "{{ postgres_database }}"
          USER: "{{ postgres_user }}"
          PASSWORD: "{{ postgres_pass }}"
          PORT: "{{ postgres_port }}"
          CONN_MAX_AGE: 0
          OPTIONS:
            sslmode: '{{ postgres_sslmode }}'

Describe alternatives you've considered
We could re-order the roles so that the likelyhood that this is hit is lower. That is what I have done for now.

Additional context

Follow-up work after #24 merges