[openstack|compute] adds methods to retireve floating & fixed ip addresses

this also refactors the public/private_ip_address methods
Merge pull request #1594 from ohadlevy/openstack-instance-ips

[openstack|compute] adds methods to retireve floating & fixed ip addresses
[digitalocean|compute] initial release

DigitalOcean Compute Service Provider.
See https://www.digitalocean.com/api
[digitalocean|compute] added getting started guide
[digitalocean|compute] Updated server model documentation
[digitalocean|compute] added test helpers
[digitalocean|compute] Implemented Server.reboot

- Added reboot_server request
- Added required tests
[digitalocean|compute] Implement Server.power_cycle

- Added power_cycle_server request
- Added required tests
[digitalocean|compute] tests updates
[digitalocean|compute] Added Servers collection tests
[digitalocean|compute] Implement missing requests

- power_off_server
- power_on_server
- shutdown_server

Added required tests
[digitalocean|compute] implemented missing Server actions

- shutdown
- stop
- start

Added required tests