refactor ownership of setting up volumes into runtime

[frantjc]

currently, the github.com/frantjc/sequence/runtime/github.com/frantjc/sequence/workflow relationship has the workflow owning the creation if the directories that get bound to the step containers. This has lead to the workflow package making some assumptions about the runtime having access to a local directory

if we desire a Kubernetes runtime (and we do) that will have to change

• a Runtime should set up its own mounts
• we can do away with the concept of mount Types e.g. bind, volume, tmpfs. We only really have 1 use for volumes

[frantjc]

resolved by https://github.com/frantjc/sequence/pull/53