Rename ClusterEnvironment.creates_processes

[awaelchli]

What does this PR do?

Part of #6303

The name/method creates_children in the cluster environment plugins is supposed to indicate whether the cluster is creating the processes or not. The naming here turned out to be confusing and ambiguous. Here is a proposal to name it more explicitly. Suggestions for alternatives welcome.

@ananthsub

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #10106 (1c99b11) into master (9b21ecb) will increase coverage by 4%.
The diff coverage is 100%.

❗ Current head 1c99b11 differs from pull request most recent head a37b71e. Consider uploading reports for the commit a37b71e to get more accurate results

@@           Coverage Diff            @@
##           master   #10106    +/-   ##
========================================
+ Coverage      89%      92%    +4%     
========================================
  Files         182      182            
  Lines       16111    16121    +10     
========================================
+ Hits        14273    14904   +631     
+ Misses       1838     1217   -621