This adds the ServiceNetMap value to global_vars for config-download.
This will make the value consumable from ansible tasks when running
config-download.
Additional values can be added in a similar fashion in the future to
allow for less hardcoded data coming out of Heat, and instead using
jinja expressions to consume data from Ansible variables when
config-download runs.
Change-Id: I8c442caac140f1c96123c1be47e858949419fd8f
e0d26441f1