Intelligent Host Selection - Clean & Cat-Log

[wxtim]

Cylc clean and cat log should use intelligent host selection.

If for example you clean platform:

[platforms]
    [[cluster]]
        hosts = foo, bar, baz

Then cylc clean and cat log should try bar or baz if it tries foo and finds it unreachable.

Pull requests welcome!

[wxtim]

@MetRonnie Did you implement/partially implement this?

[MetRonnie]

I only implemented "dumb platform selection", i.e. if clean fails due to ssh 255, try the next platform for the same install target etc.