Upgrades manifest.yml to support new inception ssh keys

It will store your existing private key in the manifest.yml and recreate `~/.bosh_bootstrap/ssh/inception` private key which is used for all SSH-related activities upon the inception VM

/cc @mrdavidlaing @rkoster can you please code review/test on any existing inception VMs/manifest.yml you have and check that it works and that your manifest.yml has been upgraded?

It automatically backs up your existing manifest.yml