Fix Webhook discovery, refresh pool before getting pool size

[leszko]

What does this pull request do? Explain your changes. (required)

Fix webhook-based discovery and remove webhook caching for Stream Tester.

Webhook-based discovery did not refresh pool while calling size() function which resulted in the following scenario:

1. For On the pool is [] (size 0)
2. Webhook is updated to On+1
3. Selection process checks the pool size and returns 0

This affected Stream Tester which returned score 0 for both On+1 and On (even though On is down and On+1 is up).

Specific updates (required)

• Refresh webhook-based pool while getting its size
• Rename env var LP_EXTEND_TIMEOUTS to LP_IS_ORCH_TESTER
• Remove webhook caching for Stream Tester

How did you test each of these updates (required)

Running Stream Tester in staging and checking the results.

Does this pull request close any open issues?

fix #2270
fix #2370
fix #2412

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[codecov]

Codecov Report

Merging #2413 (d9835b6) into master (30b7330) will decrease coverage by 0.00853%.
The diff coverage is 25.00000%.

❗ Current head d9835b6 differs from pull request most recent head 320754b. Consider uploading reports for the commit 320754b to get more accurate results

@@                 Coverage Diff                 @@
##              master       #2413         +/-   ##
===================================================
- Coverage   54.93891%   54.93038%   -0.00854%     
===================================================
  Files             93          93                 
  Lines          19316       19319          +3     
===================================================
  Hits           10612       10612                 
- Misses          8115        8118          +3     
  Partials         589         589                 

Impacted Files	Coverage Δ
common/util.go	55.70470% <ø> (ø)
discovery/wh_discovery.go	60.19417% <25.00000%> (-1.80584%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 30b7330...320754b. Read the comment docs.

[victorges]

LGTM