Add dhcp package to the CentOS repository

Add ISC DHCP package to allow usage of dhcrelay.
dhcrelay makes it possible to relay DHCP packets
into the cobbler Docker container

implements blueprint fuel-containerization-of-services

Change-Id: If286af51a1ce737d87f11e5469987f78ba64a831
Merge "Add dhcp package to the CentOS repository"