Common: Add OperatingSystems enum

Enough is enough! I have had it with these monkey-fighting strings on
this Monday to Friday plane! Everybody strap in.

Seriously, it's time to stop using strings all over the place to
identify the OS. An Enum is a better, more strongly-typed solution.

monkey/common/__init__.py

@@ -2,3 +2,4 @@
 Used for a common things between agent and island
 """
 from .di_container import DIContainer, UnregisteredTypeError
+from .operating_systems import OperatingSystems

monkey/common/operating_systems.py

@@ -0,0 +1,6 @@
+from enum import Enum
+
+
+class OperatingSystems(Enum):
+    LINUX = "linux"
+    WINDOWS = "windows"