Dev: utils: Avoid hardcoding the ssh key type as RSA

[liangxin1300]

Problem

• The join process will fail if the existing key type is not RSA. See: crmsh has issue with the default ed25519 ssh key #1504 (comment)
• If the init and join nodes are already set up for passwordless access using an ed25519 key, an RSA key pair is still generated during the init/join process.

Changes include:

• Avoid hardcoding the ssh key type as RSA
• Introduced a new function ssh_key.fetch_public_key_list to fetch public keys from local or remote, return as public key path list or public key content list
• In KeyFileManager, use class variable to store the key type instead of hardcoding it as RSA
• In bootstrap.swap_public_ssh_key function, Change the parameter name from 'add' to 'generate_key_on_remote' to make it more descriptive
• In the process of join_ssh, no need to set 'generate_key_on_remote' to True, as the key is already generated on init node
• Improve shell script in generate_ssh_key_pair_on_remote to avoid hardcoding the key type as RSA
• Remove unused code

[codecov]

Codecov Report

Attention: Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.

Project coverage is 69.26%. Comparing base (917b0d9) to head (ece148f).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
crmsh/ssh_key.py	87.50%	2 Missing ⚠️
crmsh/utils.py	33.33%	2 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
integration	54.50% <88.23%> (-0.01%)	⬇️
unit	52.13% <32.35%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
crmsh/bootstrap.py	88.63% <100.00%> (-0.04%)	⬇️
crmsh/ssh_key.py	79.76% <87.50%> (+5.39%)	⬆️
crmsh/utils.py	66.54% <33.33%> (-0.16%)	⬇️

... and 1 file with indirect coverage changes

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[nicholasyang2022]

1. It is a bad idea to return vaules in a same type with different meanings. Please use Key in function signature and use KeyFile and InMemoryKey correspondingly. Or separate these 2 different usage into 2 functions.
2. typing.List[str] is deprecated. Use list[str] instead.