Fix overcloud controller scaling

Controller scaling was broken by the commit
02772ba2877b9f6d427c6fd760bf19d6334c68a8. Merge.py raises an exception
when it tries to scale the default value "controller0" of the
`BootstrapNodeResource` parameter.

This reverts back to using Fn::Select for specifying the bootstrap host,
the rest of the Fn::Select -> get_attr changes are kept.

Change-Id: I0cdebf75d4752a35f547d4fbb81545ece3172405
Merge "Fix overcloud controller scaling"