Upgrade go version of the notebook-controller to 1.15, across the
Dockerfile, Makefile and README. We used the same Golang version as our Kubernetes
dependency, after @Jeffwan's suggestion.
* Allowing for an env var ADD_FSGROUP to be set to false to suppress the automatic addition of fsGroup: 100 in the pod's security context.
This addresses issue #4617.
* Adding note in README regarding ADD_FSGROUP.