builder/amazon/ebs: Add config for target AZ.

Fixes [GH-536]
builder/amazon/common: Add AvailZone to StepRunSourceInstance struct

[GH-536]
Merge branch 'master' of github.com:mitchellh/packer into fix#536

Conflicts:
	CHANGELOG.md
Merge pull request #550 from mitchellh/fix#536

builder/amazon/ebs: Add config for target AZ.