feat: find emulator project id from environment variable

Previously, if `FIRESTORE_EMULATOR_HOST` was set and a `Client` instance was created without an explicit project, it would fall-back to `google-cloud-firestore-emulator`

This change makes it check the `GCLOUD_PROJECT` variable for a project id first, which is used by the firebase functions emulator. This means the client will auto-detect the project id if not given in the emulator, just like in a live GCP environment

Fixes firebase/firebase-functions-python#174

Author: daniel-sanche

Diff for: google/cloud/firestore_v1/base_client.py

@@ -131,7 +131,8 @@ def __init__(
             if credentials is None:
                 credentials = AnonymousCredentials()
             if project is None:
-                project = _DEFAULT_EMULATOR_PROJECT
+                # extract project from env var, or use system default
+                project = os.getenv("GCLOUD_PROJECT", _DEFAULT_EMULATOR_PROJECT)
 
         super(BaseClient, self).__init__(
             project=project,