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