Fix "Could not connect to x: not connected" errors

[foutrelis]

Tweak SSHConnectionManager.Connect() to return a new connection if the previous one is not connected. (This occurs with expired connections.)

Maybe fixes: #56

[foutrelis]

Hi @czerwonk. Thank you for merging this fix. Please note that there is one more issue regarding connection handling where SSHConnection.close() is called on expired/closed connections (for example, when stopping the exporter with expired connections present).

This remaining issue is less annoying (as it doesn't affect the exporter's normal functioning), but if wrapping c.done <- struct{}{} in a c.isConnected() condition seems like a decent approach, I can submit a PR for it.

When I first encountered the original issue my first thought was to ensure SSHConnectionManager.connections doesn't contain expired/closed connections. But for some reason I don't readily recall, I didn't go that route and instead added the "connect if not connected" fix seen in this PR. Perhaps if my first thought isn't too hard to implement, it would fix the shutdown issue as well?