Set bootstrap_nodeid to undef by default

Set bootstrap_nodeid to undef by default

bootstrap_nodeid is a parameter looked-up with Hiera but only set on
controllers. With the transition to composable roles, we don't need to
set this parameter on compute nodes, so let's just default this value by
undef so we can deploy Neutron classes on compute nodes.

Change-Id: I94034f696c163969a7489d9cb0a20423210d3b84