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

Fixes [GH-536]

CHANGELOG.md

 ## 0.4.0 (unreleased)
 
+FEATURES:
 
+* builder/amazon/ebs: Ability to specify which availability zone to create
+  instance in. [GH-536]
 
 ## 0.3.10 (October 20, 2013)
 

builder/amazon/common/run_config.go

@@ -23,6 +23,7 @@ type RunConfig struct {
 	SubnetId             string `mapstructure:"subnet_id"`
 	TemporaryKeyPairName string `mapstructure:"temporary_key_pair_name"`
 	VpcId                string `mapstructure:"vpc_id"`
+	AvailZone            string `mapstructure:"avail_zone"`
 
 	// Unexported fields that are calculated from others
 	sshTimeout time.Duration
@@ -83,6 +84,7 @@ func (c *RunConfig) Prepare(t *packer.ConfigTemplate) []error {
 		"subnet_id":               &c.SubnetId,
 		"temporary_key_pair_name": &c.TemporaryKeyPairName,
 		"vpc_id":                  &c.VpcId,
+		"avail_zone":              &c.AvailZone,
 	}
 
 	for n, ptr := range templates {

builder/amazon/ebs/builder.go

@@ -101,6 +101,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 			SourceAMI:          b.config.SourceAmi,
 			IamInstanceProfile: b.config.IamInstanceProfile,
 			SubnetId:           b.config.SubnetId,
+			AvailZone:          b.config.AvailZone,
 			BlockDevices:       b.config.BlockDevices,
 		},
 		&common.StepConnectSSH{

website/source/docs/builders/amazon-ebs.html.markdown

@@ -120,6 +120,9 @@ Optional:
 * `vpc_id` (string) - If launching into a VPC subnet, Packer needs the
   VPC ID in order to create a temporary security group within the VPC.
 
+* `avail_zone` (string) - Destination availability zone to launch instance in.
+  Leave this empty to allow Amazon to auto-assign..
+
 ## Basic Example
 
 Here is a basic example. It is completely valid except for the access keys: