Implement block storage APIs for DigitalOcean

[adamwg]

Implement block storage APIs for DigitalOcean

Description

DigitalOcean has a new block storage feature in beta. The API docs are available here. This PR implements the block storage API calls in the DigitalOcean driver.

A few things to keep in mind at this point:

• Block storage is currently only available to users enrolled in the beta. Block storage API calls will fail for other users.
• Block storage is not yet available in all DigitalOcean regions.
• DigitalOcean block storage does not support snapshots, so snapshot-related APIs are still unimplemented.

Status

Done, ready for review.

Checklist (tick everything that applies)

• Code linting (required, can be done after the PR checks)
• Documentation
• Tests
• ICLA (required for bigger changes)

[tonybaloney]

Cool, great contribution! LGTM 👍 for adding tests too

[adamwg]

Thanks!

FYI, there is a breaking change to the DO block storage API coming in the next couple weeks (like I said, the feature is in beta). If you'd like to merge this now I'll submit a follow-up PR when the API change happens, or if you'd like to hold off I'll update this PR after the change.

[adamwg]

The breaking API change I mentioned went live this afternoon, so I've just pushed a new version of my branch that complies with the new API. We're not expecting the API to change again, so it should be safe to merge this any time.

Let me know if there are any changes I should make, or some more process I need to go through to get this merged :-).

[tonybaloney]

Thanks @adamwg changes look good. merging.