Convert Fn::Select to extended get_attr

Overcloud bootstrap_nodeid is now specified by parameter
BootStrapNodeResource with default value controller0.
This avoids the need to use Fn::Select on the mergy.py
built list of controllers to specify the first controller.

Change-Id: Id9cfeab50b90ceeeae51ea0e35997b7495b28cc4
Partial-Blueprint: tripleo-juno-remove-mergepy