Fix sceptre_user_data resolvable properties calls

[ngfgrant]

Context

Given a stack bundle/b/y.yaml

template_path: vpc.yaml
parameters:
  CidrBlock: 10.0.0.0/16
sceptre_user_data:
  test_block: !stack_output bundle/c/z.yaml::CidrBlock
  S3_Version: !S3Version
  S3_Key: 456
  S3_Bucket: 123

# s3_resolver.py
...
bucket, bucket_key = [
            self.stack.sceptre_user_data.get("S3_Bucket"),
            self.stack.sceptre_user_data.get("S3_Key")
        ]
...

When S3Version Resolver calls a key (S3_Bucket) on a ResolvableProperty (sceptre_user_data) sceptre will enter into a recursion and fail. If the Resolver that is called is called does not reference a ResolvableProperty then sceptre will execute normally.

Suggested fix

It won't be ideal, but adding a class that stores the global state of all ResolvableProperties by namespace would probably solve this issue. When a ResolvableProperty is called it will first check the cache for the state.

States:

"unresolved"
"resolving"
"resolved"

If "resolved" the value will be used from the cache. Else trigger the property to be resolved ("unresolved" -> "resolving") or wait for the value to be resolved ("resolving" -> "resolved").

class ResolverCache():
    def __init__(self):
        self.cache = {}

    def add(self, stack, property, key, value):
        self.cache.add(
            "{}.{}.{}.{}".format(stack, property, key):
            {
                "state": ""
                "value": value
            }

Would result in a object like

{
    "bundle/b/y.sceptre_user_data.S3_Key": 
       {
        "state": "resolved",
        "value": "456"
       }
}

We could/should additionally add an option for users to update/skip the cache so that the value is fresh each time.

There is a good chance we will need to add a DAG to manage the dependencies of Resolvers so that we don't have circular dependencies between ResolvableProperties.