add configuration checking to confd template

[olafmol]

the current (094) VGA implementation uses confd but doesn't do any specific validating of the generated haproxy configuration before (re)loading it. We should add this step:

https://github.com/kelseyhightower/confd/blob/master/docs/template-resources.md#example

http://stackoverflow.com/questions/39609178/validate-haproxy-cfg

(NB haproxy should be run in quiet mode to disable warnings)