Add VALUE_AUTO and VALUE_GENERATE constants

'auto' is used in multiple place in the code with a meaning similar to
'default', 'generate' will be used to force generation of mac addresses
when the default would be to reuse an old one.

Forward-port-of: r1884, Reviewed-by: iustinp

Reviewed-by: imsnah

lib/constants.py

@@ -141,6 +141,8 @@ XEN_INITRD = _autoconf.XEN_INITRD
 KVM_PATH = _autoconf.KVM_PATH
 
 VALUE_DEFAULT = "default"
+VALUE_AUTO = "auto"
+VALUE_GENERATE = "generate"
 VALUE_NONE = "none"
 
 # hooks-related constants