This reverts commit 898155df.

The default value of the ironic configuration option default_boot_option
will eventually change from netboot to local. The netboot option is
incompatible with multitenancy in ironic, as it requires a PXE
environment in the tenant network, so it was no longer deemed a suitable
default value.

Ironic added a warning message when this option is not explicitly set,
presumably to alert operators to the change. The commit being reverted
set the option to 'netboot'. This will cause operators to continue to
use the netboot option even after the default value changes, which was
presumably not the intention of the ironic team in changing the default
value. It also hides the warning message from the operator that could
alert them to the fact that this default is changing.

Change-Id: I0ebb1d5ffbead50b034488337e6c93a2f48aaf69
Related-Bug: #1696636