SerializableConfigParser: Make Loads class indep

Currently SerializableConfigParser.Loads is a static method that returns
a SerializableConfigParser. With this patch we change it to a class
method that returns a member of the class. This way a subclass calling
Loads on itself will get its own member, rather than a bare
SerializableConfigParser.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>