Ensure redis and sentinel services are running

During testing of our Redis setup I killed the processes via (`ps aux | grep [r]edis | awk '{ print $2 }' | xargs sudo kill`). When I re-ran the playbook I expected it to ensure services were started.

I opted for a separate task (as opposed to adding `state=started` to the start at boot task) to improve readability in Ansible output :smile: