Add clean build flag

Adds an attribute to the STI build strategy to always run a clean build.
Assuming every client has to know about the strategy that makes sense.

----- Original Message -----
> > @@ -150,6 +150,9 @@ type DockerBuildStrategy struct {
> >  type STIBuildStrategy struct {
> >  	// BuilderImage is the image used to execute the build.
> >  	BuilderImage string `json:"builderImage,omitempty"
> >  	yaml:"builderImage,omitempty"`
> > +
> > +	// Clean flag forces the STI build to not do incremental builds if true.
> > +	Clean bool `json:"clean,omitempty" yaml:"clean,omitempty"`
> 
> I think it's specific, at least from a naming perspective....The docker build
> strategy can have its own flag, named whatever we want (cache=false seems
> logical to me as a default).  Trying to extract it out to a common name
> doesn't make sense since as soon as we add a 3rd strategy it may not apply
> again anyway.
> 
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> https://github.com/openshift/origin/pull/362/files#r20109849