Fix Sentinel.execute_command to execute across the entire sentinel cluster

[SyedTahaA]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

closes: #2458

Sentinel.execute_command fixed to execute across sentinel cluster when once argument is not given or set to false, or execute on one random sentinel from the cluster if the once argument is true (like the documentation suggests).

[codecov-commenter]

Codecov Report

Base: 92.22% // Head: 92.20% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (8b66098) compared to base (fa45fb1).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2459      +/-   ##
==========================================
- Coverage   92.22%   92.20%   -0.03%     
==========================================
  Files         113      113              
  Lines       29239    29239              
==========================================
- Hits        26967    26961       -6     
- Misses       2272     2278       +6     

Impacted Files	Coverage Δ
redis/sentinel.py	86.16% <100.00%> (ø)
tests/test_cluster.py	96.86% <0.00%> (-0.23%)	⬇️
tests/test_asyncio/test_cluster.py	97.36% <0.00%> (-0.13%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.