reintroduce wait_for_pxe_state to fix node state transistions

This function was added in the branch release/essex-hack-suse/master.
As the node state transitions break with slow admin nodes, we need to
wait for the next state to be set in the pxe configuration
Merge pull request #173 from jdsn/release/pebbles/master-wait-for-pxe

reintroduce wait_for_pxe_state to fix node state transistions with slow admin nodes