param lookup race condition

[rebeccaskinner]

first identified with blackbox/test_graph_generation.sh in samples/platform.sh with the error

2016-09-19T12:16:41-05:00 |WARN| setting session-local token    token=32a7a76a-7028-43e2-ba96-0fba3b2f02d7
2016-09-19T12:16:41-05:00 |INFO| serving    addr=:47740 component=rpc
2016-09-19T12:16:41-05:00 |INFO| resolving dependencies component=rpc function=ResolveDependencies runID=eb7c1bfb-46ad-4983-a97f-e06f9d622c7b
2016-09-19T12:16:41-05:00 |INFO| loading resources  component=rpc function=SetResources runID=eb7c1bfb-46ad-4983-a97f-e06f9d622c7b
2016-09-19T12:16:41-05:00 |ERROR| could not render  component=rpc error=template: destination:1:18: executing "destination" at <param `filename`>: error calling param: param not found location=/Users/rebecca/go/src/github.com/asteris-llc/converge/samples/platform.hcl runID=eb7c1bfb-46ad-4983-a97f-e06f9d622c7b
2016-09-19T12:16:41-05:00 |ERROR| loading failed    component=rpc error=rendering /Users/rebecca/go/src/github.com/asteris-llc/converge/samples/platform.hcl: template: destination:1:18: executing "destination" at <param `filename`>: error calling param: param not found function=grapher.Graph runID=eb7c1bfb-46ad-4983-a97f-e06f9d622c7b
2016-09-19T12:16:41-05:00 |FATAL| could not get graph   component=client error=error getting graph component: rpc error: code = 2 desc = loading failed: rendering /Users/rebecca/go/src/github.com/asteris-llc/converge/samples/platform.hcl: template: destination:1:18: executing "destination" at <param `filename`>: error calling param: param not found file=/Users/rebecca/go/src/github.com/asteris-llc/converge/samples/platform.hcl
rebecca@rmbp2:~/go/src/github.com/asteris-llc/converge$ cat samples/platform.hcl

Suspected race condition related to graph ordering and/or param lookup.

[rebeccaskinner]

Fixed in #265