Handle case when SSH key is stored in Mdm::Cred#pass

[MSP-11041](https://jira.tor.rapid7.com/browse/MSP-11041)

This prevents a migration error when Pro decides to store an SSH key in Mdm::Cred#pass, instead of storing the path.

Verification:
- [ ] In Pro 4.9.3, save an Mdm::Cred somewhere with `ptype: 'ssh_key'`, and `pass: File.read('/path/to/your/key')`. You can do this through the console or by going to a Host -> Credentials -> Add Cred, type=ssh private key and pasting in a cred.
- [ ] Upgrade to 4.10.0, which migrates your DB
- [ ] Migration should not fail