Update preallocated hosts to use Passphrase credentials

Summary: Depends on D7695.  This updates preallocated hosts to use Passphrase credentials.  Due to the way SSH private key text credentials work (the TempFile disappears before SSH commands can be executed), this only supports file-based private keys at the moment.

Test Plan:
Created a Passphrase credential for a file-based SSH key.  Allocated a resource with:

```
bin/drydock create-resource --blueprint 1 --name "My Linux Host" --attributes platform=linux,host=localhost,port=22,path=/var/drydock,credential=2
```

and successfully leased it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4111, T1049

Differential Revision: https://secure.phabricator.com/D7697