Add monitoring flag for Digital Ocean droplets

[TheYorkshireDev]

Digital Ocean now has the functionality for more extensive monitoring of droplets, which includes the ability to set alerts for droplets. These new features only work if a monitoring script is run on the droplet upon startup, fortunately the Digital Ocean API allows for a flag to be passed through (similar to how private networking is switched on) that pre-installs the monitoring script allowing the more in-depth monitoring. I have simply added this flag to docker-machine.

Testing:

1. Manually tested the updated help text

Using:

./docker-machine create --driver digitalocean --help

Output:

...
   --digitalocean-ipv6								enable ipv6 for droplet [$DIGITALOCEAN_IPV6]
   --digitalocean-monitoring							enable monitoring for droplet [$DIGITALOCEAN_MONITORING]
   --digitalocean-private-networking						enable private networking for droplet [$DIGITALOCEAN_PRIVATE_NETWORKING]
...

2. Manually created a droplet using the flag and verified the monitoring script was detected in the Digital Ocean UI

Using (in ./bin/):

./docker-machine create --driver digitalocean --digitalocean-access-token xxxxx --digitalocean-private-networking=true --digitalocean-monitoring=true docker-sandbox

Fixes: #4227

Signed-off-by: Steven Cooney dev@stevenjamescooney.com

[dgageot]

LGTM