[bug] improve safety of policy strategy parsing

[jrasell]

The strategy block of a scaling policy is opaque to Nomad and therefore does not go through validation during job registration. It is the responsibility of the Autoscaler to parse the policy object safely without panics.

Currently the strategy parsing makes some assumptions about the objects underlying types, including that the operator always includes the name parameter within the strategy block. This is a false assumption, and can result a panic when this param is excluded.

panic: interface conversion: interface {} is nil, not string

goroutine 47 [running]:
github.com/hashicorp/nomad-autoscaler/policystorage.parseStrategy(0x16df8e0, 0xc0003c81a0, 0x180e209)
	/Users/jrasell/go/src/github.com/hashicorp/nomad-autoscaler/policystorage/nomad.go:128 +0x31d
github.com/hashicorp/nomad-autoscaler/policystorage.(*Nomad).Get(0xc000692008, 0xc0002d2180, 0x24, 0x194b700, 0xc00000e048, 0x0)
	/Users/jrasell/go/src/github.com/hashicorp/nomad-autoscaler/policystorage/nomad.go:60 +0x308
github.com/hashicorp/nomad-autoscaler/agent.(*Agent).Run.func1(0xc0007143a0, 0x195bea0, 0xc000134480, 0xc000692008, 0x1966320, 0xc0005640f0, 0xc000256570, 0xc0002d2180, 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