(#12884) Use default ssh.private_key_path when ssh.keys_only = false

[wbclark]

Per config.ssh documentation, ssh.private_key_path should use the default key path when not set. ssh.keys_only = false as worded does not prevent key based auth, it simply does not limit to key based auth; setting ssh.keys_only = false should not change the default behavior for ssh.private_key_path.

Fixes #12884

[hashicorp-cla]

All committers have signed the CLA.

[wbclark]

Thanks very much for the quick triage, review, and merge.