Enable pxe_network_config option.

This option tells the nova baremetal driver to append the network
configuration to the kernel cmdline when pxe booting the baremetal
image. Initramfs built with network support could then benefit from
this option and avoid the need of having the network configuration file
injected into the image.

Change-Id: I757dbb8224a9e0eee64da4ee71da5567b18b3892
Merge "Enable pxe_network_config option."