Avoid echoing comment from makefile recipe

Avoid echoing comment from makefile recipe

We include a comment to explain why nova-compute-instance.yaml is
included in the prerequisite list for overcloud.yaml, but this comment
gets echoed by make before it executes the recipe using a shell.

  $> make overcloud.yaml
  # overcloud-source.yaml [..] won't work here because [..]
  # a prerequisite but don't want to pass it into merge.py
  python ./tripleo_heat_merge/merge.py overcloud-source.yaml [..]
  mv overcloud.yaml.tmp overcloud.yaml

Instead, maintain a list of files included by overcloud.yaml in a
variable and put a comment above that.

Change-Id: I1af69fbb33e1833170f086aec9b54c5c4e57138f