Digitalocean driver: Support for creating Droplets with Cloud-init User Data

[janeczku]

This PR enhances the Digitalocean driver to support bootstrapping droplets with Cloud-init User Data.
It adds a new create flag which allows users to pass a file containing User Data (e.g. cloud-config or shell script) to the driver.

New flag

Path to user-data file

• EnvVar: DIGITALOCEAN_USERDATA
• Name: digitalocean-userdata

Example usage for deploying CoreOS machine

$ cat <<EOF >>/tmp/cloud-config.yaml
#cloud-config
coreos:
  etcd2:
    discovery: https://discovery.etcd.io/ABC123456
    advertise-client-urls: http://$private_ipv4:2379,http://$private_ipv4:4001
    initial-advertise-peer-urls: http://$private_ipv4:2380
    listen-client-urls: http://0.0.0.0:2379,http://0.0.0.0:4001
    listen-peer-urls: http://$private_ipv4:2380
  units:
    - name: etcd2.service
      command: start
    - name: fleet.service
      command: start
EOF

$ docker-machine create -d digitalocean digitalocean-access-token ABC123456 \
--digitalocean-image coreos-stable --digitalocean-private-networking \
--digitalocean-userdata /tmp/cloud-config.yaml test-droplet

[janeczku]

Related PR #1947

[dgageot]

Hi @janeczku any chance you could rebase this PR?

[janeczku]

@dgageot Sure np!

[janeczku]

@dgageot rebased & updated docs

[nathanleclaire]

LGTM

[dgageot]

LGTM