Introduce a "volume" field set

Based on issue #440 and this related discussion https://github.com/elastic/ecs/pull/441#discussion_r279478675, I think having a "volume" field set would be beneficial.

Just like `host` or `cloud`, it could be used as an optional set of metadata about volumes where activity is being observed, or even as a first class field set, if someone wants to inventory volumes with Elasticsearch.

Here are some ideas for fields that could go in there:

- name, keyword
- filesystem, keyword
- size, long (bytes)
- used, long (bytes)
- letter, keyword: Windows drive letter
- kind, keyword: e.g. ssd, magnetic
- type, keyword: platform specific types, such as AWS' gp2, io1 etc
- id, keyword
- serial_number, keyword

Additional ideas & points of view welcome.