[bug] parsing policy source and query can result in panic

[jrasell]

The top level params within a scaling policy are opaque to Nomad and therefore does not go through validation during job registration. It is the responsibility of the Autoscaler to parse the top level params safely without panics.

Currently the autoscaler assumes the source and query params are key/value string parameters. This is fine if the policy is correctly forms, but results in a panic if the operator has made a configuration error.

Example:

source = {
  name = "prometheus"
 }

Panic:

panic: interface conversion: interface {} is []interface {}, not string

goroutine 66 [running]:
github.com/hashicorp/nomad-autoscaler/policystorage.(*Nomad).Get(0xc0004e6008, 0xc000017410, 0x24, 0x10000000194b020, 0x1000000000001, 0x0)
	/Users/jrasell/go/src/github.com/hashicorp/nomad-autoscaler/policystorage/nomad.go:50 +0x552
github.com/hashicorp/nomad-autoscaler/agent.(*Agent).Run.func1(0xc00088a050, 0x195b800, 0xc000092580, 0xc0004e6008, 0x1965c20, 0xc0007a0180, 0xc000031920, 0xc000017410, 0x24)
	/Users/jrasell/go/src/github.com/hashicorp/nomad-autoscaler/agent/agent.go:84 +0xa6
created by github.com/hashicorp/nomad-autoscaler/agent.(*Agent).Run
	/Users/jrasell/go/src/github.com/hashicorp/nomad-autoscaler/agent/agent.go:78 +0x60f