OHAI-445: Detect ecdsa ssh host keys

(Open)SSH supports 3 types of elliptic curve keys: ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384 and ecdsa-sha2-nistp521. To be consistent with the file
name and the normal abbreviation the key itself is stored as
[:keys][:ssh][:host_ecdsa_public]. [:keys][:ssh][:host_ecdsa_type] specify
the subtype of ecdsa. This field is needed, to generate known_host files.
OHAI-445: Fallback to default HostKey path

This fixes a bug in the ssh_host_key plugin that prevented the plugin
from reading ssh_host_*_key files when a sshd_config exists.

When a sshd_config exists but the HostKey directive is not present, i.e.
using the default value, the plugin should populate the host_*_public
attributes using the default HostKey locations.