Fix #93 - context not being set on StepTypeBootCommand

[thebinary]

The issue is due to empty context being set as config.ctx in the builder. So the interpolate function used inside StepTypeBootCommand has no knowledge of the UserVariables.

This commit fixes the issue and so the issue #93

[joncave]

Just submitted #97 to fix this in a slightly cleaner way by setting the InterpolateContext. This is how the core Hyper-V builder works: https://github.com/hashicorp/packer/blob/master/builder/hyperv/iso/builder.go#L108-L116. The Decode function calls DecodeContext already and will set the context data on the passed InterpolateContext: https://github.com/hashicorp/packer/blob/master/helper/config/decode.go#L45-L48.