For others' reference, setting up an SSH Key credential and setting it as the "Default Preferred Host Credential" is explained well (with screenshots) in the following MOS article:
How to set SSH-key Named Credential as preferred credential (Doc ID 1453530.1)
A similar approach works for using SSH Key-based credential for the "Database Host Credential", except that the credential set has an auth_target_type (host) that is different from the target_type (oracle_database). The reason seems to be that the credential is associated with a database instance, but is used to authenticate against a host. The code below works...
$ ./emcli create_credential_set -set_name="SSH-Key DB Host Credentials" -target_type=oracle_database -supported_cred_types=HostSSHCreds -auth_target_type=host -description="SSH Key-based DB Host Credentials"
Credential set "SSH-Key DB Host Credentials" created successfully.
Once that is done, you can select the new credential set as an alternative to "Database Host Credentials"