r/promtail: Optionally run with DAC_READ_SEARCH

The *promtail* service runs as an unprivileged user by default, which is
fine in most cases (i.e. when scraping only the Journal), but may not
always be sufficient to read logs from other files.  Rather than run
Promtail as root in these cases, we can assign it the
CAP_DAC_READ_SEARCH capability, which will allow it to read any file,
but does not grant it any of root's other privileges.

To enable this functionality, the `promtail_dac_read_search` Ansible
variable can be set to `true` for a host or group.  This will create a
systemd unit configuration extension that configures the service to have
the CAP_DAC_READ_SEARCH capability in its ambient set.

roles/promtail/defaults/main.yml

@@ -1,3 +1,5 @@
+promtail_dac_read_search: false
+
 promtail_positions_file: /tmp/positions.yaml
 
 promtail_clients: