SSH public keys are configurable

Currently there can only be one active SSH public key per server
instance. Changing the public key requires a file change and reboot.

Add new sshkey query parameter for use when generating CoreOS iPXE boot
scripts. SSH public keys live under
`$COREOS_IPXE_SERVER_DATA_DIR/sshkeys` in a file named ${sshkeyid}.pub.

Fixes issue 1.