Add clean build flag

Adds an attribute to the STI build strategy to always run a clean build.
Doesn't matter whether its implementation is different - would there be an interface to the user that needs "clean"?  By adding "clean" you are implicitly stating that not all builds are clean.

----- 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"`
> 
> Hmm... I guess we could think of it as generic. It's definitely accomplished
> differently in STI vs Docker builds though. @bparees - what do you think?
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/openshift/origin/pull/362/files#r20080747