In order for glean to operate properly and regenerate ssh host keys it
needs to start on a host without any ssh host keys. We hadn't been doing
this meaning that new host keys were not generated on images using
simple-init. Fix this by having simple-init delete any host keys found
in /etc/ssh late in the build process.
This should force glean/sshd to generate new host keys on first boot
depending on the distro.
Change-Id: Ic917387d92febf21c2cc37dfc1fa83fcf0c8e469
