[WIP] Add base image from the build UpstreamImage if set

A rough in progress change.



> On Dec 17, 2014, at 12:02 PM, Ben Parees <notifications@github.com> wrote:
> 
> In pkg/build/api/types.go:
> 
> > @@ -37,6 +37,8 @@ type BuildParameters struct {
> >  
> >  	// Output describes the Docker image the Strategy should produce.
> >  	Output BuildOutput `json:"output,omitempty" yaml:"output,omitempty"`
> > +
> > +	UpstreamImage string `json:"upstreamImage,omitempty" yaml:"upstreamImage,omitempty"`
> Yes, we know we're going to have to refactor this PR once the dust settles on my changes, so don't worry about the exact parameter/config definition here.
> 
> the image change triggers are going to work the same way the deployment ones work today, which means a reference to an imagerepository (imagestream).
> 
Right, but they should identify what pull spec in the build strategy they work on as well.  Image repo name doesn't have to equal pull spec in strategy.  Indirection here allows the ability to work with sources under different control
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