[28.x backport] Deprecate special handling for quoted values for TLS flags

[thaJeztah]

• backport Deprecate special handling for quoted values for TLS flags #6290

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relates to:

• opts: deprecate QuotedString #6274
• docker 1.13.0 CLI seems to break previous flag parsing behavior moby/moby#29761

The --tlscacert, --tlscert, and --tlskey command-line flags had non-standard behavior for handling values contained in quotes (" or '). Normally, quotes are handled by the shell, for example, in the following example, the shell takes care of handling quotes before passing the values to the docker CLI:

docker --some-option "some-value-in-quotes" ...

However, when passing values using an equal sign (=), this may not happen and values may be handled including quotes;

docker --some-option="some-value-in-quotes" ...

This caused issues with "Docker Machine", which used this format as part of its docker-machine config output, and the CLI carried special, non-standard handling for these flags.

Docker Machine reached EOL, and this special handling made the processing of flag values inconsistent with other flags used, so this behavior is deprecated. Users depending on this behavior are recommended to specify the quoted values using a space between the flag and its value, as illustrated above.

- What I did

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- Human readable description for the release notes

Deprecate special handling for quoted values for the `--tlscacert`, `--tlscert`, and `--tlskey` command-line flags

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[codecov-commenter]

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[thaJeztah]

Flaky test;

=== Failed
=== FAIL: e2e/registry TestOauthLogin (1.01s)
    login_test.go:28: terminating PID 9678
    login_test.go:33: assertion failed: expression is false: strings.Contains(string(output), "USING WEB-BASED LOGIN"): Failed to start web-based login - falling back to command line login...
        
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