builder/digitalocean: add a droplet_name configuration value

Practically, this lets you set the hostname of the droplet
used for provisioning.
builder/virtualbox: no panic if ssh host port min/max is same [GH-594]
builder/vmware: force LANG=C for ifconfig [GH-592]
builder/vmware: check for ifconfig in /sbin [GH-591]
Merge pull request #589 from mitchellh/f-do-droplet-name

builder/digitalocean: add a droplet_name configuration value
Update CHANGELOG
v0.3.11