Fix loading of keys and create repo when old yubikey flag is used

[renatav]

Description (e.g. "Related to ...", etc.)

When loading signing keys, we want to first try reading from keystore files, then from the inserted Yubikey, before asking the user to do anything. The problem was that the check if key was already read from a YubiKey was not properly implemented, so the same key was getting read twice. Also tried to improve the overall user experience when loading keys.
Found issues with creating a repository in case when old yubikey flag was used to specify that a role's metadata files should be signed using YubiKeys.

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• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[n-dusan]

I believe this should return None, since in taf/keys.py L307, L308 there is a check for None