Replace hard-coded template name by the file name

[BenFradet]

https://github.com/snowplow/dataflow-runner/blob/master/src/config_resolver.go#L210

t, err := template.New("playbook").Funcs(templFuncs).Parse(string(rawBytes))

generates an error that might be misleading.

e.g. if someone forgets to specify a template variable in cluster.json, the generated error will be:

template: playbook:1:10: executing "playbook" at <.var>: map has no entry for key "var"

[BenFradet]

I reckon the best template name would be the filename, what do you think @alexanderdean ?

[alexanderdean]

Yep agree! (Wow the original impl is confusing)

[BenFradet]

I just noticed it while writing the blog post :smile: