Promtail-friendly one line logs

[raneq]

Print log entries as single lines, also in error case. This implies wrapping the execution of commands in case they fail, and printing their output flattened (\n escaped) in that case.

3 examples:

Fail Output Example #1

2019-12-23 15:13:01.982148376+00:00 | INFO | Create temporal directory
2019-12-23 15:13:01.985607707+00:00 | INFO | Postgres DB dump
2019-12-23 15:13:02.278978954+00:00 | INFO | Compress assets data
2019-12-23 15:13:02.289908150+00:00 | INFO | Restic backup
2019-12-23 15:13:03.107826457+00:00 | ERROR | Fatal: unable to open repo at b2:some-bucket:/: b2.NewClient: b2_authorize_account: 401:

Fail Output Example #2

2019-12-23 15:43:01.162415961+00:00 | INFO | Create temporal directory
2019-12-23 15:43:01.165357643+00:00 | INFO | Postgres DB dump
2019-12-23 15:43:01.471210237+00:00 | INFO | Compress assets data
2019-12-23 15:43:01.481858818+00:00 | INFO | Restic backup
2019-12-23 15:43:03.052475062+00:00 | ERROR | Fatal: unable to open repo at b2:non-existing-bucket:/: Bucket: non-existing-bucket: bucket not found

Success Output

2019-12-27 09:27:01.222474096+00:00 | INFO | Create temporal directory
2019-12-27 09:27:01.229189669+00:00 | INFO | Postgres DB dump
2019-12-27 09:27:02.238105299+00:00 | INFO | Compress assets data
2019-12-27 09:27:02.252841960+00:00 | INFO | Restic backup
2019-12-27 09:27:18.586371329+00:00 | INFO | Restic forget
2019-12-27 09:27:24.489343955+00:00 | INFO | Restic check
2019-12-27 09:27:34.028960454+00:00 | INFO | Backup complete

I also create a template config file for promtail that I have checked it works. However, it's not backups-role purpose to install and configure promtail, so we don't do anything with that config file. I leave it there more as reference. We could move it to a wiki page or /docs dir.

[sauloperez]

I'd like to have the link to the PR as well. In the vast majority of cases we'll be reading the Changelog from our computers and having the link will come in handy.