Fix _parse_value always returning 0 when not true, false or None.

system/sysctl

@@ -187,7 +187,7 @@ class SysctlModule(object):
             return ''
         elif value.lower() in BOOLEANS_TRUE:
             return '1'
-        elif not value.lower() in BOOLEANS_FALSE:
+        elif value.lower() in BOOLEANS_FALSE:
             return '0'
         else:
             return value.strip()