Remove support to legacy YAML format in serialized collections

Cobbler has already been using JSON for a long time, so legacy
YAML format is not needed anymore.
Improve header doc in serializers
Rename directory with serialized Cobbler objects from config to collections
Drop .d in collections directories

.d is a common suffix in Linux to refer to configuration directories
in which configuration in split in several files. Collections are
*not* configuration files, they are data created by user, so this
nomenclature does not make sense.
Rename Config class to Collection Manager

Configuration is already used in Cobbler with several other meanings:
boot loader configuration file, DHCP server configuration file,
Cobbler configuration file. What this class does is managing
collections of Cobbler objects and serializing/deserializing them.
Class name should make it clearer to avoid confusion.
Merge pull request #1218 from alanoe/alanoe/rename_config

alanoe/rename config