Rename pxe_template_dir setting to boot_loader_conf_template_dir

PXE refers to DHCP+TFTP. This template directory only contains
boot loader configuration templates, so setting name should reflect
that more clearly.
Rename source/dest boot loader configuration templates directories

PXE refers to DHCP+TFTP. This template directory only contains
boot loader configuration templates, so directory name should
reflect that more clearly.
Rename file serializer/deserializer name

Old name was serializer_catalog.py. Catalog is not an intuitive name,
serializer_file.py is more intuitive.
Merge pull request #1243 from jmaas/master

Docs. Style change for release-notes.
Merge pull request #1215 from alanoe/alanoe/rename_boot_loader_conf_templates_dir

alanoe/rename boot loader conf templates dir
Merge pull request #1217 from alanoe/alanoe/rename_catalog_serializer

Rename file serializer/deserializer name
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