Issue 1753: Add support for DescribeSecret

- Merge changes from upstream master.
- Update get_secret_value to use helper method for validating secret
identifier.
- Update implementation coverage checklist.

moto/secretsmanager/models.py

@@ -47,7 +47,7 @@ class SecretsManagerBackend(BaseBackend):
 
     def get_secret_value(self, secret_id, version_id, version_stage):
 
-        if secret_id not in (self.secret_id, self.name):
+        if not self._is_valid_identifier(secret_id):
             raise ResourceNotFoundException()
 
         response = json.dumps({