Rename/update common/tftp.py to common/pxe_utils.py

The functions in the common/tftp.py are not related to TFTP, the
functions are PXE related and most of them are used to generate the
PXE configuration file. Moving it from tftp.py to pxe_utils.py will make
it less confusing and is also needed by the iPXE work since iPXE doesn't
generate any configuration file under the TFTP folder.

The dhcp_options_for_instance() method was updated to not receive the PXE
boot file image from the parameters and just get it from the configuration
because we were always using the value from the configuration anyway.

Implements: blueprint ipxe-boot
Change-Id: Icfbaa4d6cdb9cbe0c392bf3dbd50ad980c70560c
Merge "Rename/update common/tftp.py to common/pxe_utils.py"