The kernel paramter 'initrd' is hard-coded as 'deploy_ramdisk' in iPXE
config template. A rescue ramdisk may fail to boot due to this wrong
paramter. This patch sets this 'initrd' paramter properly to
'deploy_ramdisk' or 'rescue_ramdisk' by adding a new pxe option.
Change-Id: I8ad1223dc289bbe915ee4ca8db66c8f80bf08f15
Closes-Bug: 1749860