Introduce per-profile/per-system next-server support.
Make DHCP configuration to use per-profile/per-system next-server.
Rename PXEGen class to TFTPGen

PXE refers to DHCP + TFTP. PXEGen name seems that this class would
generate DHCP conf file as well and it does not; this is confusing.
Rename class to make it clear.
Rename BootAPI to CobblerAPI
Rename BootCLI to CobblerCLI
Merge pull request #1202 from r0ckpine/feature/per-system-next-server

Introduce per-profile/per-system next-server
Merge pull request #1214 from alanoe/alanoe/rename_pxegen

Rename PXEGen class to TFTPGen
Merge pull request #1216 from alanoe/alanoe/rename_boot_classes

alanoe/rename boot classes
Docs: updates to the release-notes.
Docs: updates to the release-notes.
Merge branch 'master' of github.com:jmaas/cobbler
Merge pull request #1219 from jmaas/master

Docs. Updates to the release-notes.
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.