[api] refactoring: caching directory for root user is now /var/cache/bauh

bauh/api/user.py

@@ -1,5 +1,6 @@
 import os
+from typing import Optional
 
 
-def is_root():
-    return os.getuid() == 0
+def is_root(user_id: Optional[int] = None):
+    return user_id == 0 if user_id is not None else os.getuid() == 0