cmd/puppeth: make it possible to have pw-protected keyfiles

[holiman]

This PR makes the ssh connection sequential. This is important in the case where user feedback is needed, e.g. when the ssh key is password protected.

With this PR:

INFO [01-08|11:24:53.385] Administering Ethereum network           name=rinkeby
INFO [01-08|11:24:53.387] Dialing previously configured server     server=karalabe:id_rsa_2017@boot.rinkeby.io
What's the decryption password for /home/user/.ssh/id_rsa_2017? (won't be echoed)
>
INFO [01-08|11:25:00.430] Dialing previously configured server     server=karalabe:id_rsa_2017@web.rinkeby.io
What's the decryption password for /home/user/.ssh/id_rsa_2017? (won't be echoed)
>
INFO [01-08|11:25:06.580] Starting remote server health-check      server=karalabe:id_rsa_2017@web.rinkeby.io
INFO [01-08|11:25:06.580] Starting remote server health-check      server=karalabe:id_rsa_2017@boot.rinkeby.io

Without this PR:

What's the decryption password for /home/user/.ssh/id_rsa_2017? (won't be echoed)
>What's the decryption password for /home/user/.ssh/id_rsa_2017? (won't be echoed)
>

The health-check-phase is still performed in parallel.