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 
Since these are now in the `var` block in main, you can use the less-verbose direct declaration syntax, i.e. instead of

```go
var (
    debug bool
)
flag.BoolVar(&debug, "debug", false, "debug logging")
```

you can do

```go
var (
    debug = flag.Bool("debug", false, "debug logging")
)
```

This works for everything except `configuredVolumes`, which is a special type.