Fix bug #367726 by reverting some default user handling introduced while fixing bug #256612.
* bzrlib/transport/ssh.py: (_paramiko_auth): Explicitly use getpass.getuser() as default user.
* bzrlib/transport/ftp/_gssapi.py: (GSSAPIFtpTransport._create_connection): Explicitly use getpass.getuser() as default user.
* bzrlib/transport/ftp/__init__.py: (FtpTransport._create_connection): Explicitly use getpass.getuser() as default user.
* bzrlib/tests/test_sftp_transport.py: (TestUsesAuthConfig.test_sftp_is_none_if_no_config) (TestUsesAuthConfig.test_sftp_doesnt_prompt_username): Revert to None as the default user.
* bzrlib/tests/test_remote.py: (TestRemoteSSHTransportAuthentication): The really offending one: revert to None as the default user.
* bzrlib/tests/test_config.py: (TestAuthenticationConfig.test_username_default_no_prompt): Update test (and some PEP8).
* bzrlib/smtp_connection.py: (SMTPConnection._authenticate): Revert to None as the default user.
* bzrlib/plugins/launchpad/account.py: (_get_auth_user): Revert default value handling.
* bzrlib/config.py: (AuthenticationConfig.get_user): Fix doc-string. Leave default value handling to callers.