Split SSH and SFTP credentials in test suite configuration

Currently, the SSH and SFTP tests use the same remote login name and credentials. This makes things needlessly complicated for implementing #9142 (closed). Please decouple these two things, e.g. add (and use) a SFTP section in the config file, with the same format as the existing SSH one.

Feature Branch: kytv:test/9263-split-ssh-variables

Related issues

Blocks #9142 (closed)

Original created by @intrigeri on 9263 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information