Configurable retry count for SSH authentication

[bluekeyes]

i'm using giraffe for password-based SSH auth
and i want to try to connect 3 times before giving up
is there a way to do this nicely?
(specifically, i'm trying to make the try-3-times part clean)

This will probably an environment option in the map passed to newFileSystem / newExecutionSystem, which means that that map needs to be more obvious for users. Consider this when redesigning how to get HostControlSystems in #15.