Add k8s-env --dev option

This PR makes the following changes:

Adds --dev option to k8s-env command
- This flag when enabled will add the entire teraslice repo as a volume to allow for fast build changes when running in kubernetes.
Adds dev compatibility to k8s master backend
Fixes asset-storage setting in k8s-env
- This resolves a bug where launching k8s-env with asset-storage set to s3 would not work correctly

terascope / teraslice