Allow customization of SSH Key pair pattern

Defined in a template using ssh_keypair_pattern. Defaults to "packer %s"
Enable customization of VirtualBox disk controller with `hard_drive_interface` option (SATA only).
Document the `hard_drive_interface` option.
Merge pull request #391 from jbronn/hard_drive_interface

builder/virtualbox: Add `hard_drive_interface` option
Update CHANGELOG
builder/virtualbox: process hdd interface as template, validate
Merge pull request #389 from whostolebenfrog/master

builder/amazon/ebs: Allow customization of temporary ssh key name
Update CHANGELOG
packer: uuid function
Update CHANGELOG
builder/amazon/all: TemporaryKeyPairName

/cc @whostolebenfrog - I actually simplified things quite a bit. I added
a "uuid" global template function so it just uses that now. I renamed it
so that it is clear it is a temporary keypair.
Merge branch 'master' into sysresc

Conflicts:
	builder/virtualbox/step_create_disk.go