Add unknown OS type constant.

coriolis/constants.py

@@ -95,6 +95,7 @@ OS_TYPE_OS_X = "osx"
 OS_TYPE_SOLARIS = "solaris"
 OS_TYPE_WINDOWS = "windows"
 OS_TYPE_OTHER = "other"
+OS_TYPE_UNKNOWN = "unknown"
 
 DEFAULT_OS_TYPE = OS_TYPE_LINUX