remove the metadata filter in the sandbox (#63)

## Description
Using a custom endpoint instead of using the default endpoint that `hegel` adjusts 
<!--- Please describe what this PR is going to change -->

## Why is this needed
The reason behind that is, while using the sandbox in combination with tinkerbell's example [workflows](https://github.com/tinkerbell/workflows). The [functions.sh](https://github.com/tinkerbell/workflows/blob/master/ubuntu_18_04/00-base/functions.sh) will fail due to lack of information in the retrieved metadata from `hegel`. The default endpoint filters out all the needed metadata such as `plan_slug`. This PR removes that filtration criteria. 

Also I am not sure, I think we are safe to provide these info(the full hardware spec) to the worker while using the Sandbox setup, as this is mainly used as an example setup not as a production one.   
<!--- Link to issue you have raised -->

Fixes: #64 

## How Has This Been Tested?
<!--- Please describe in detail how you tested your changes. -->
<!--- Include details of your testing environment, and the tests you ran to -->
<!--- see how your change affects other areas of the code, etc. -->
Yes, it was tested locally by setting the env var in the docker-compose.yml file and batch it in the sandbox setup. 

## How are existing users impacted? What migration steps/scripts do we need?

<!--- Fixes a bug, unblocks installation, removes a component of the stack etc -->
<!--- Requires a DB migration script, etc. -->


## Checklist:

I have:

- [ ] updated the documentation and/or roadmap (if required)
- [ ] added unit or e2e tests
- [ ] provided instructions on how to upgrade

deploy/docker-compose.yml

@@ -165,6 +165,7 @@ services:
       TINKERBELL_GRPC_AUTHORITY: 127.0.0.1:42113
       TINKERBELL_CERT_URL: http://127.0.0.1:42114/cert
       DATA_MODEL_VERSION: 1
+      CUSTOM_ENDPOINTS: '{"/metadata":""}'
     depends_on:
       db:
         condition: service_healthy