fix for different username on remotes bug

Fixes a bug whereby a user with a different username on a remote machine was incorrectly ssh'ing to that machine using their current username regardless of settings in their .ssh/config file.

So for user "bob" who has a username of "bob" on his desktop but "bob123" on his hpc he would normally put the following in his .ssh/config file:

```
Host hpc*
    User bob123
```

For a task:
```
[[foo]]
    [[[remote]]]
        host = hpc
```
Currently cylc will do its operations as `ssh bob@hpc` whereas it should be using `ssh bob123@hpc` or, in actuality, `ssh hpc` in the absence of any overrides. While there is the functionality for bob to write:

```
[[foo]]
    [[[remote]]]
        host = hpc
        owner = bob123
```
This would be undesirable in a shared suite as whoever ran it would have to change the owner setting to their username each time they ran it.

This change should fix the problem.