qa: Default to tty=False for SSH

This fixes commands over SSH, such as `AssertCommand()`,
putting both stderr and stdout of the remote command into the
stdout of the SSH, which led to `AssertCommand()` seeing different
output than the invoked commands actually produce.

The `tty` option was originally added to *disable* `-t` (see commits
50265802 and f14a8b15), so we can as well make that the default.

Signed-off-by: Niklas Hambuechen <niklash@google.com>