add _common.py with run_on_all_deployments for migration scripts

`run_on_all_deployments` can be used byother migration scripts, causing them to be run on all production deployments
It does this by scanning the `nrel-openpath-deploy-configs` repo for a list of deployments and modifying the DB_HOST accordingly in between each time the function is called

bin/historical/migrations/_common.py

@@ -0,0 +1,41 @@
+import os
+import subprocess
+import importlib
+import logging
+
+import emission.core.get_database as edb
+
+DB_HOST_TEMPLATE = "mongodb://localhost:27017/openpath_prod_REPLACEME"
+
+proc = subprocess.run(
+      'rm -rf nrel-openpath-deploy-configs && ' +
+      'git clone --no-checkout https://github.com/e-mission/nrel-openpath-deploy-configs.git && ' +
+      'cd nrel-openpath-deploy-configs && ' +
+      'git ls-tree -r --name-only HEAD | grep configs/',
+      shell=True,
+      capture_output=True,
+      text=True)
+filenames = proc.stdout.replace("configs/", "").split("\n")
+
+PROD_LIST = [
+    fname.split(".")[0]
+    for fname in filenames
+      if fname and 'dev-' not in fname and 'stage-' not in fname
+]
+print(f"PROD_LIST: {PROD_LIST}")
+
+
+def run_on_all_deployments(fn_to_run):
+    """
+    Run the given function on the database for each deployment by setting the
+      DB_HOST environment variable in between each function call.
+    The list of deployments (PROD_LIST) is retrieved from the
+      nrel-openpath-deploy-configs repo upon initialization of this module.
+    """
+    for prod in PROD_LIST:
+        prod_db_name = prod.replace("-", "_")
+        print(f"Running {fn_to_run.__name__} for {prod} on DB {prod_db_name}")
+        os.environ['DB_HOST'] = DB_HOST_TEMPLATE.replace(
+            "REPLACEME", prod_db_name)
+        importlib.reload(edb)
+        fn_to_run()