Fixes the following vet reports:

builder/digitalocean/builder_test.go:267: arg b.config.SSHUsername for printf verb %d of wrong type: string
builder/digitalocean/builder_test.go:300: arg b.config.RawSSHTimeout for printf verb %d of wrong type: string
builder/digitalocean/builder_test.go:341: arg b.config.RawStateTimeout for printf verb %d of wrong type: string
builder/digitalocean/builder_test.go:382: arg b.config.PrivateNetworking for printf verb %s of wrong type: bool
builder/digitalocean/builder_test.go:397: arg b.config.PrivateNetworking for printf verb %s of wrong type: bool