1. add packages(xinetd,tftpd,tftp) to ubuntu pre-install list, they
are required by the latest ironic devstack plugin.
2. remove "n-novnc" service disable option, it's included by following
nova disable options.
3. add some descirption to clarify that core services(Horizon, Nova, Cinder)
don't must be disabled.
4. remove heat disable option, heat is disabled by default, make the
options be minimal.
5. add IRONIC_TEMPEST_WHOLE_DISK_IMAGE=False, otherwise the Iroinc tempest
would be failed, because default ironic agent deploy driver require
partition image, should keep consistent with tempest.
6. remove IRONIC_VM_EPHEMERAL_DISK=0, it's same with default value.
7. remove network options, it's not neccessary, keep the beginner focus
on the Nimble related options, and keep the demo local.conf clean and
simple.
Change-Id: Ie389c50c394d2a52938899fc7d2e75dce6167933
295eeb8bea