Fix overcloud controller scaling

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