fix: allow an environment variable to default the deploy namespace
fix: allow an environment variable to default the deploy namespace

lets allow an environment variable to be used to define the helm deploy namespace
fixes #496

GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold