boot_mode is overwritten in node properties/capabilities when secure boot is requested for iLO drivers

When deploy is requested for by setting instance_info/capabilities with secure_boot='true', the code oevrwrites the capabilities in the node/properties.

Steps to reproduce:-
$ ironic node-update $NODE remove properties
$ ironic node-update $NODE add instance_info/image_source=$UBUNTU_CLOUD_IMAGE instance_info/root_gb=5
$ ironic node-update $NODE add instance_info/capabilities='{"secure_boot":"true"}'
$ ironic node-set-provision-state $NODE active

$ ironic node-show $NODE | grep properties
| properties | {u'capabilities': u'boot_mode:uefi'}

This defect applies to all iLO drivers.

It should not update the 'boot_mode' in properties/capabilities, but should internally take it as the uefi boot mode.

This could lead to failure if the boot mode that gets set internally happens to be BIOS when secure boot is requested by the user.