Ruby 2.4 nightly builds

[hallelujah]

Running nightly:

• mysql + ruby 2.4
• postgres + ruby 2.4
• oracle + ruby 2.4

Normal PR are still running on ruby 2.3

Replaces #790
Closes THREESCALE-2062

Note to reviewers.

All nightly builds will run first, if green I will just activate the triggers at midnight.
I will squash some commits too

The commit named "Fix ArgumentError: key must be 32 bytes..." could be extracted to another PR

• Make every workflow green:
  

• Docker build on 2.4

• Nightly builds on 2.4

[codecov]

Codecov Report

Merging #814 into master will decrease coverage by 0.66%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #814      +/-   ##
==========================================
- Coverage    92.9%   92.24%   -0.67%     
==========================================
  Files        2414     2340      -74     
  Lines       78355    75119    -3236     
==========================================
- Hits        72797    69293    -3504     
- Misses       5558     5826     +268

Impacted Files	Coverage Δ
...ib/three_scale/spam_protection/checks/timestamp.rb	97.95% <100%> (ø)	⬆️
...ce_discovery/import_cluster_definitions_service.rb	32.3% <0%> (-67.7%)	⬇️
app/lib/stats/views/active_clients.rb	33.33% <0%> (-66.67%)	⬇️
app/lib/three_scale/email_sanitizer.rb	25% <0%> (-66.67%)	⬇️
...b/events/importers/first_daily_traffic_importer.rb	37.5% <0%> (-62.5%)	⬇️
app/helpers/google_experiments_helper.rb	28.57% <0%> (-57.15%)	⬇️
test/test_helpers/events.rb	45.45% <0%> (-54.55%)	⬇️
lib/developer_portal/lib/liquid/wrapper.rb	45.45% <0%> (-50%)	⬇️
app/workers/janitor_worker.rb	50% <0%> (-50%)	⬇️
app/workers/report_traffic_worker.rb	53.33% <0%> (-44.45%)	⬇️
... and 132 more

---

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[Martouta]

Ok, I asked Hery and he said:

because openssl on ruby 2.4 is not accepting more than 32 bytes secret