Allow different parts of a project to manage a config file

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Consider a config file such as /etc/hosts.deny which contains access controls for network daemon processes. Such a file, on a given host, could conceivably need to be updated by different parts of a project, for example, a module that hardens NFS and one that hardens SSH.

There should be means for Droid to:-

• specify the contents of a platform-dependant config file, perhaps using per-host variables
• write the config file to the host
• allow targets and modules to depend on the config file being in place, perhaps by way of per-host tasks that are run before any other targets are allowed to be run.