Avoid high load for Cobbler TFTP when delete many nodes.

Delete operation does not have limit for nodes, but it
erase disks and reboot nodes after which they try to boot
using network. Cobbler installed at master node provide
ability to boot using network and have limited resources.

To prevent high load for Cobbler this changes add two things:
* split all nodes to groups (default - 10) which process
in series;
* wait some time (default - 10 sec) between such groups.

Both of this parameters can be changed in config.

Closes-Bug: #1330938
Closes-Bug: #1339024

Backport from 5.1

Change-Id: I9c3af6e8ab3c7c610e31baa6e58ec86aae20708d