do reset the env values when --reset is used

When I use `cf push --reset` with a manifest I noticed that the environment variables were not re-applied as defined in the manifest.

This patch does apply the environment variables as defined in the manifest with a `cf push --reset`.

This PR was originally made against the vmc codebase in https://github.com/cloudfoundry/manifests-vmc-plugin/pull/2
@austinbv suggested I port it to cf.