fix(providers): update dop calls

Before dop v0.1.6, the call to get all SSH keys listed on your
account was called all_ssh_keys(). In v0.1.6, they introduced a
couple of breaking changes, including the change to rename
all_ssh_keys() to ssh_keys().

ref: https://github.com/ahmontero/dop/blob/v1.6/dop/client.py#L196
Merge pull request #691 from opdemand/fix-digitalocean

fix(providers): update dop calls