Templating for git hooks

Currently, pre-receive hook is fixed: https://github.com/hasura/gitkube/blob/master/build/gitkubed/pre_receive.sh

We want to make it scriptable to enable : https://github.com/hasura/gitkube/issues/20

Each hook probably needs variables from the remote like DEPLOYMENTS, REGISTRY_PREFIX.
For pre-receive, we are currently doing a moustache render during startup.

For giving same variables to each hook, we need to run a moustache render on each hook and then place it in the correct directory.

Start with build/gitkubed/pre_receive.sh and the existing moustache render performed during startup. Trace how hooks are created and placed, then verify that each hook receives remote variables such as DEPLOYMENTS and REGISTRY_PREFIX and is rendered into the correct directory.

Written by the indexing model from the issue text.