Avoid resetting netboot flag unless requested

Currently the netboot enabled flag is being set for all nodes with
every Puppet run on the build server. Change the parameter in
cobbler.yaml from a global default to a per-node option and document
the expected behavior:

 - nodes added with no flag specified are set to netboot by default
 - nodes with no flag specified will not have their netboot option
   in cobbler changed by subsequent build node Puppet runs
 - nodes with a flag specified will have that flag applied by Puppet
   on every run of Puppet on the build node

Closes-Bug: #1273453