feat: add more timeout configuration on screenshots

[dpgaspar]

SUMMARY

Fixes error handling on screenshots and adds more configuration options around screenshots timeout

New configuration keys:
SCREENSHOT_SELENIUM_HEADSTART: in seconds
SCREENSHOT_SELENIUM_RETRIES: in seconds

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[willbarrett]

Change looks good - do we want to add tests for the additional options and error handling?

[codecov]

Codecov Report

Merging #14868 (fd97c34) into master (0e07a5c) will decrease coverage by 0.00%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master   #14868      +/-   ##
==========================================
- Coverage   77.64%   77.64%   -0.01%     
==========================================
  Files         966      966              
  Lines       49615    49616       +1     
  Branches     6311     6311              
==========================================
  Hits        38524    38524              
- Misses      10890    10891       +1     
  Partials      201      201              

Flag	Coverage Δ
hive	?
mysql	81.70% <80.00%> (?)
postgres	81.73% <80.00%> (+0.06%)	⬆️
presto	81.41% <80.00%> (?)
python	81.99% <80.00%> (-0.01%)	⬇️
sqlite	81.34% <80.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/utils/webdriver.py	78.08% <75.00%> (+24.02%)	⬆️
superset/config.py	91.15% <100.00%> (+0.06%)	⬆️
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	70.32% <0.00%> (-17.08%)	⬇️
superset/utils/core.py	88.91% <0.00%> (-0.13%)	⬇️
superset/views/core.py	75.48% <0.00%> (+0.44%)	⬆️
superset/models/core.py	90.00% <0.00%> (+1.05%)	⬆️
superset/connectors/sqla/models.py	89.57% <0.00%> (+1.18%)	⬆️
superset/db_engine_specs/mysql.py	94.28% <0.00%> (+1.42%)	⬆️
superset/db_engine_specs/presto.py	89.89% <0.00%> (+5.47%)	⬆️
... and 1 more

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