--private-key fixes

I ran into issue #3 (even though I have ssh-agent running fine).  So I created a passphrase-less ssh key to use with bosh-bootstrap, and pointed to it with the --private-key option.  That got me further, but I ran into problems later on in the bootstrap process due to the specified key not being used.  These commits fix those problems, so that the process completes successfully for me.