containertool: Rename --username/password to --default-username/password

[euanh]

Motivation

The --username and --password options define default values which are used if no suitable credentials can be found in .netrc. Their current names could imply that they override other sources of credentials - the help originally said this was the case (#123). Renaming them to --default-username and --default-password reduces the risk of misunderstanding.

Modifications

• Rename the --user and --password options to --default-user and --default-password.
• Add checks to warn that the old versions are deprecated, and fail if both are provided together.
• Hide the old options from --help and remove them from the manual page
• Rename the environment variable versions of these options. The environment variable change had not yet been released so deprecation warnings are not needed.

Result

• New users will see more descriptive option names
• Existing users will be prompted to change the options they pass

Test Plan

All existing test continue to pass.