Make SENTRY_DSN an optional env var

Why these changes are being introduced:

There has long been some friction with the SENTRY_DSN env var.  In a production setting
we ideally do not want an application to get deployed without the SENTRY_DSN set.  At
the same time, it's really not operationally required for most applications.  It's
probably the wrong ergonomics to have it checked generally and always at the config
level, when it's more of a deployed check.

How this addresses that need:
* make SENTRY_DSN optional
* issues a WARNING log message when unset or explicit "None" value

Side effects of this change:
* An application could get deployed without a SENTRY_DSN
* We can, and should, look at deploy practices that will enforce
SENTRY_DSN being present

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/IN-1542

Author: ghukill

lambdas/config.py

@@ -9,11 +9,8 @@
 
 
 class Config:
-    REQUIRED_ENV_VARS = (
-        "WORKSPACE",
-        "SENTRY_DSN",
-    )
-    OPTIONAL_ENV_VARS = ("WARNING_ONLY_LOGGERS",)
+    REQUIRED_ENV_VARS = ("WORKSPACE",)
+    OPTIONAL_ENV_VARS = ("WARNING_ONLY_LOGGERS", "SENTRY_DSN")
 
     def check_required_env_vars(self) -> None:
         """Method to raise exception if required env vars not set."""
@@ -103,4 +100,4 @@ def configure_sentry() -> None:
             "Sentry DSN found, exceptions will be sent to Sentry with env=%s", env
         )
     else:
-        logger.info("No Sentry DSN found, exceptions will not be sent to Sentry")
+        logger.warning("No Sentry DSN found, exceptions will not be sent to Sentry")

tests/test_config.py

@@ -23,6 +23,12 @@ def test_config_doesnt_configure_sentry_if_dsn_not_present(caplog, monkeypatch):
     assert "No Sentry DSN found, exceptions will not be sent to Sentry" in caplog.text
 
 
+def test_config_doesnt_configure_sentry_if_dsn_is_explicit_none(caplog, monkeypatch):
+    monkeypatch.setenv("SENTRY_DSN", "None")
+    configure_sentry()
+    assert "No Sentry DSN found, exceptions will not be sent to Sentry" in caplog.text
+
+
 def test_config_missing_required_env_vars(monkeypatch):
     monkeypatch.delenv("WORKSPACE")
     with pytest.raises(