boot_devices.PXE value should match with pyghmi define

pyghmi ipmicmd.get_bootdev() returns 'network' for pxe boot mode. However
ipminative defines 'net' as the name, so it does not match with pyghmi
return value.

In this patch, we redefine boot_devices.PXE value to 'network' which match
pyghmi.

Change-Id: Ic19597d2e577098a08e5ca2c5c6f0660200fc1cb
Closes-Bug: 1397212
Merge "boot_devices.PXE value should match with pyghmi define"