fix: collab-server in kubernetes not working due to hardcoded graphql_engine hostname

[sebastienbaillet]

Identify the Bug

To fix #794

Description of the Change

Allow graphql_engine hostname customization in collab-server deployment, to allow Ghostwriter to be fully operational when deployed in Kubernetes.
To do so, you can set GRAPHQL_HOST env var in collab-server deployment to match graphql_engine service name.

Alternate Designs

We may have graphql_engine change to something RFC1123 compliant everywhere, but it's way more easier to allow to set graphql_engine hostname in collab-server.

Possible Drawbacks

There should be none, as the default behavior (not setting the env var) is the same as before.

Verification Process

I've tested the modification in a kubernetes cluster.

Release Notes

Allow graphql_engine hostname customization in collab-server deployment