Remove global variables from harpoon-agent.

I'm getting a permissions error on one test, which is causing it to fail:

```
--- FAIL: TestAgentHostResources (0.00 seconds)
	api_test.go:165: HTTP 500 (mkdir all /run/harpoon/123: mkdir /run: permission denied)
```

All other tests are passing. When you have a chance we can talk about what is happening.

Addresses #116