This adds a Meta field (similar to InstanceState.Meta) to InstanceDiff.
This allows providers to store arbitrary k/v data as part of a diff and
have it persist through to the Apply. This will be used by helper/schema
for timeout storage being done by @catsby.
The type here is `map[string]interface{}`. A couple notes:
* **Not using `string`**: The Meta field of InstanceState is a string
value. We've learned that forcing things to strings is bad. Let's
just allow types.
* **Primitives only**: Even though it is type `interface{}`, it must
be able to cleanly pass the go-plugin RPC barrier as well as be
encoded to a file as Gob. Given these constraints, the value must
only comprise of primitive types and collections. No structs,
functions, channels, etc.
This is the first step in allowing overrides of map and list variables.
We convert Context.variables to map[string]interface{} from
map[string]string and fix up all the call sites.
This is REALLY heavy and would be really hard to maintain any sort
of compatibility with, but it is what we're going to do during dev
initially (if we don't ship with it) in order to just get stuff working.
Mitchell Hashimoto