[OHAI-402] Correct defaults for host_#{type}_public (HostKey)

The `ssh_host_key` plugin must set one or more of the
`host_ecdsa_public`, `host_rsa_public`, or `host_dsa_public`
attributes, based on the following logic:

 - If `HostKey` is explicitly set in `sshd_config`,
   each of the specified public key files must be read,
   the key type detected, and the corresponding attribute
   must be set.
 - If either `HostKey` was not explicitly set in `sshd_config`,
   or the file itself does not exist, then use the OpenSSH
   (protocol version 2) defaults:
   - `/etc/ssh/ssh_host_dsa_key`, `/etc/ssh/ssh_host_ecdsa_key`,
     and `/etc/ssh/ssh_host_rsa_key`.
   - for all the default files that were found on disk, set
     the corresponding Ohai attribute.
 - OpenSSH 5.7 and later support ECDSA (RFC5656), so this
   must be supported as well, without breaking earlier versions
   of OpenSSH.