Skip unchanged vars when updating config

[nn-bhowes]

I am using a direct docker image deployment method, but this seems to always cause a redeployment, which is not desirable.

The root cause is that if APTIBLE_DOCKER_IMAGE appears in the config map (see: https://github.com/aptible/go-deploy/blob/master/aptible/app.go#L121), it triggers a deployment rather than a config update.

I've created this code as a complete Go n00b, but wanted to demo what would be a useful solution to me: not setting the unchanged variables. In my case this means that the unchanged APTIBLE_DOCKER_IMAGE key will get dropped and a configuration change will be applied rather than a deployment in DeployApp (as far as I can tell)

If this solution is not acceptable, one alternative which would also be useful is to have a sentinel value for APTIBLE_DOCKER_IMAGE, which when used will avoid the redeployment