Added dynamic_startup_nodes configuration to RedisCluster.

[barshaul]

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

Added dynamic_startup_nodes configuration to RedisCluster.
By default, when reinitializing the cluster topology, the nodesManager will use only the initial passed startup nodes to refresh the cluster topology. If dynamic_startup_nodes is set to true, the nodesManager will set the startup nodes to all of the discovered nodes and they will be used to determine the cluster nodes-slots mapping in the next topology refresh.

When true, the initial passed startup nodes will be removed if their endpoints aren't listed in the CLUSTER SLOTS output. For example, if dynamic DNS endpoints are used for startup nodes, but CLUSTER SLOTS lists specific IP addresses, the passed startup nodes will be overridden with the specific nodes' IP.
Therefore, it is recommended to keep dynamic_startup_nodes set to false with DNS endpoints.

[codecov-commenter]

Codecov Report

Merging #2244 (1ba23fe) into master (6da8086) will decrease coverage by 13.41%.
The diff coverage is 18.18%.

@@             Coverage Diff             @@
##           master    #2244       +/-   ##
===========================================
- Coverage   91.85%   78.44%   -13.42%     
===========================================
  Files         108      108               
  Lines       27776    27786       +10     
===========================================
- Hits        25515    21797     -3718     
- Misses       2261     5989     +3728     

Impacted Files	Coverage Δ
redis/cluster.py	19.09% <0.00%> (-71.22%)	⬇️
tests/test_cluster.py	18.58% <25.00%> (-78.49%)	⬇️
tests/test_asyncio/test_cluster.py	18.92% <0.00%> (-78.53%)	⬇️
redis/asyncio/cluster.py	16.92% <0.00%> (-73.00%)	⬇️
redis/asyncio/parser.py	17.30% <0.00%> (-63.47%)	⬇️
redis/commands/cluster.py	32.10% <0.00%> (-62.11%)	⬇️
redis/crc.py	45.45% <0.00%> (-54.55%)	⬇️
redis/utils.py	75.67% <0.00%> (-13.52%)	⬇️
... and 10 more

---

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 6da8086...1ba23fe. Read the comment docs.

[dvora-h]

@barshaul tests fails after merge master in.

[barshaul]

@barshaul tests fails after merge master in.

taking a look.