Use ssh.config.SSHConfig to expand hostnames

[thekorn]

Since a couple of days [0] the ssh.config module has support for variable expansion. Once this has been released in the ssh library, fix should support it too, such that something like

$ fix nodes:dc1,dc2,dc3

automatically expands to

$ fix nodes:dc1.boo.org,dc2.boo.org,dc3.boo.org

If configured in ~/.ssh/config, or any alternative location.

[0] https://github.com/bitprophet/ssh/commit/31482a46d6c360e00284a7ccfd68362891ab316b