OHAI-445: Detect ecdsa ssh host keys

Current versions of (Open)SSH supports in addition to dsa and rsa ecdsa (elliptic curve) keys. Ohai should extract this host keys, too.

(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_hosts files.