Fix bugs in Ansible playbook that caused `make dev-go` to fail

[eloquence]

• When the grsec kernel is not present, the previous logic caused a default Node image (without the custom CI Dockerfile) to be used. As a result, npm install would fail (due to missing python and g++ dependencies) and make dev-go would time out at this stage.

• The SHA-256 hash was not appended to the name of the Django image. Once the bug above was fixed, this caused Docker to look for an image called stn_django:latest (latest is the default tag). Since that image does not exist, it would fail at this stage.

[conorsch]

Great catch, @eloquence. Admittedly I don't test much on non-grsec kernels, so mistakes like this one can slip in. We've made similar updates to other Django-based repos we're working on. @msheiny can you take a look here?

Confirmed the changes presented here work fine on a grsec-patched kernel.

[eloquence]

Glad to help, really like the new dev environment overall :)