Wait for successful ssh login, not just ssh available

#### Description

Rather than just waiting for the SSH server to be available, it would be better to allow it to be available but wait until the login is successful.

#### Use Case(s)

I want my installation environment (i.e. Anaconda/kickstart with EL) to enable ssh during it's execution so that issues with it can be debugged. Packer currently sees this ssh as meaning the install is complete however it's not of course. Packer fails to authenticate to that ssh instance though, so continuing to wait for an ssh authentication that is successful would allow for this use-case.

No files, tests, or entry points are named. Start by locating Packer's SSH readiness and authentication flow, then trace where an available SSH server is treated as ready. Done means provisioning continues only after login succeeds, with tests covering a server that accepts connections but rejects authentication.

Written by the indexing model from the issue text.