crond: add support for crontab file only (on any OS)

[jkellerer]

Implements #288

The PR allows to use an external scheduler that supports crontab format when configured as:

global:
  scheduler: "crontab: /path/to/crontab/file"

(this is available on any OS)

The existing crond support using the crontab binary remains unchanged for linux

waiting on #259 before implementing remaining tests

[creativeprojects]

This would also help with #321 as I realised that the crond implementation in busybox (alpine linux) has funny rules when trying to run as a non-root user. It doesn't work with the current implementation of using crontab to set or update the scheduling.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 78.00000% with 66 lines in your changes are missing coverage. Please review.

Project coverage is 71.51%. Comparing base (85d5afc) to head (957db95).

Files	Patch %	Lines
crond/mock/main.go	0.00%	34 Missing :warning:
schedule/handler_crond.go	48.00%	11 Missing and 2 partials :warning:
crond/io.go	87.32%	5 Missing and 4 partials :warning:
crond/crontab.go	91.78%	3 Missing and 3 partials :warning:
schedule/handler.go	86.67%	2 Missing :warning:
schedule/scheduler_config.go	95.56%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
+ Coverage   71.07%   71.51%   +0.44%     
==========================================
  Files         121      121              
  Lines       12394    12659     +265     
==========================================
+ Hits         8809     9053     +244     
- Misses       3189     3201      +12     
- Partials      396      405       +9     

Flag	Coverage Δ
unittests	71.51% <78.00%> (+0.44%)	:arrow_up:

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[jkellerer]

Had tested all options manually, think it is ready now.

[jkellerer]

Yes. Supersonic is one option to have a compatible task scheduler that is OS independent (besides that it needs to run as service)

[jkellerer]

The use case with alpine should work as well. Had used busybox’s cron with custom crontab files previously.

[creativeprojects]

The use case with alpine should work as well. Had used busybox’s cron with custom crontab files previously.

Yes it works when running as root. With this PR I managed to get resticprofile scheduled as a non-privileged user (using Supercronic). I'll add a contrib for it.

I'm also tempted to provide another docker image with running resticprofile as a non-privileged user.