Add 2-sec sleep between slave nodes start

There are some issues either with iPXE or
dhcrelay which lead to the problem with
PXE booting of virtual slaves. This
workaround makes bootstrapping success
more likely

Change-Id: I85584f61cd06a7c5cd1f73e6638c9475d0f86ac2
Partial-Bug: #1317213
(cherry picked from commit 81348bd8c8f7cba312ff76872ec1ec464967acd2)
Relocation for jenkins jobs

Change-Id: Idda03f2c06a7b29911e2003983424ad5ba68f9dd