[WIP] Follow GitLab Upstream on GKE Marketplace

- Check out GitLab submodule from upstream repository instead of special
  forked repository
- Create a wrapper chart for the Google GKE Marketplace named gitlab-mp
  allowing special configuration unique to the marketplace to be passed
  in without having to change the upstream chart
- Add env-doctor script allowing users to quickly verify that their
  environment is ready to produce a GKE Marketplace update
- Fortify list-helm-images, update-schema-yml, mirror-helm scripts to
  work when invoked from any path
- Remove a deprecated script 'build.sh'
- Move the Dockerfile definition to the top level directory and simplify
  to use the GKE Marketplace deployer_helm_tiller build container
- Move schema.yaml to schema.yaml.template and have the build script
  make a copy and run update-schema-yml against the copy instead of the
  original to prevent unwanted modifications from being committed to the
  repository
- Add build-marketplace-release.sh script to build the deployer
  container from the configured environment
- Added tooling to push builds to the GKE Marketplace and build and
  tear down a test environment
- Modify update-schema-yaml.sh to also insert RBAC information
- Updated documentation; moved original README to doc/installing and
  split documentation into user versus developer

Resolves: charts/gitlab#1041

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>