Factoring out PXE and TFTP functions

Multiple drivers need to support neutron and Ironic's PXE and TFTP
server. Factoring them out into a common file will make that
transition possible. This moves all the generic calls into neutron
and reconfigures the PXE driver to work with the new functions.

Change-Id: I181ffabe673636b628329592e623e6425da54d6d
Merge "Factoring out PXE and TFTP functions"