Support Listing Droplets by Tag

[mjtieman]

Digital Ocean's V2 API supports listing droplets by tag name, docs here. Supporting this API will allow users to perform tag-based droplet discovery.

[chavenor]

This is linked to #ansible/ansible#25105

[chavenor]

Where are we at with the release of Dopy 0.3.7 which is holding up the code merge with ansible to enable tags?