Update boot-stack element's README.
Update boot-stack element's README.

Remove some outdated instructions, which have by now
been better realized in incubator/notes.md, and document
the first-boot process, and boot-stack specific command
line utilities.

Change-Id: I50950ea1411e1ad57dabaf3f6667f7fd5c45d6be

qwefi/tripleo-image-elements,qwefi/tripleo-image-elements,rdo-management/tripleo-image-elements,radez/tripleo-image-elements,agroup/tripleo-puppet-elements,openstack/tripleo-image-elements,qwefi/tripleo-image-elements,agroup/tripleo-puppet-elements,qwefi/tripleo-image-elements,openstack/tripleo-image-elements,agroup/tripleo-puppet-elements,qwefi/tripleo-image-elements,rdo-management/tripleo-image-elements,agroup/tripleo-puppet-elements,radez/tripleo-image-elements