Title: Be compatible to old site=None setting read from hosts.mk
Level: 1
Component: wato
Compatible: compat
Version: 1.2.8b10
Date: 1459951352
Class: fix

Old WATO versions saved the site attribute with a value of None which
is not allowed in current WATO versions. Either the key has to be missing
when no specific site is configured or the key has to be missing. Cleaned
this up.

Fixed exception: TypeError (cannot concatenate 'str' and 'NoneType' objects)
