add support for install in LXC containers

[siavashsardari]

Running Redis inside LXC containers requires Redis config which basically tell systemd that the service is running in an unprivileged env and cannot use private devices.

[Migsi]

@DavidWittman please consider merging this change as this is solves a major problem with this role.

[rromanchuk]

@siavashsardari @Migsi i have a semi related issue, just in terms of needing to be able to modify the systemd template. I like the idea of having opinionated defaults, but i can't really think of a good solution to expose modifications i need. Exposing more systemd variables seems like a never ending task. Like in my case, i also need to define specific After= targets as well.

This seems a bit rigid, but even if there was an option to provide your own template, it would basically invalidate/NOOP role variables which seems dangerous.

- name: create redis systemd service
  template:
    src: "{{ item }}"
    dest: /etc/systemd/system/{{ redis_service_name }}.service
    mode: 0644
  with_first_found:
    - files:
      - "{{ ansible_os_family }}/redis.service.j2"
      - default/redis.service.j2
      paths:
        - ../templates
  register: redis_unit_file
  when: redis_as_service and ansible_service_mgr|default() == "systemd"