Add default environment override back to common.mk (#1586)

* As part of https://github.com/chanzuckerberg/cellxgene/pull/1548 we
  accidentally removed the part of the "get_or_else_dev_env_default"
  function that allowed users to override the environment variables.
  This commit adds that back.
* When environment.default was changed from a shell script file to json,
  the shell commands used to parameterize DATASET were not evaluated.
  This commit fixes this issue as well.

environment.default.json

@@ -2,7 +2,7 @@
   "CXG_CLIENT_PORT": 3000,
   "CXG_OPTIONS": "--debug",
   "CXG_SERVER_PORT": 5005,
-  "DATASET": "$(git rev-parse --show-toplevel)/example-dataset/pbmc3k.h5ad",
+  "DATASET": "example-dataset/pbmc3k.h5ad",
   "DEBUG": "debug",
   "DEV": "dev",
   "JEST_ENV": "prod",