The delay required for downloading tftp boot image

The delay allows to increase time for obtaining a boot image by
fuel-slave nodes and decreasing CPU load on fuel-master node.

Change-Id: I1da48a2374c4d4f0783b12bb9f736e54098fcb41
Closes-Bug: #1401851
(cherry picked from commit 65a9bcdbd228dc8478091999689c7c81bcd7d762 )
Merge "The delay required for downloading tftp boot image" into stable/6.0