feat(Makefile): let each project manage how it builds

[bacongobbler]

Instead of hardcoding docker build -t deis/<component> ., we should let each project manage how they build themselves by handing it over to their make build command.

[mboersma]

My concern is that if we do allow container projects leeway over what make build does, it will be harder to integrate with deis/tester, which benefits from the current standardization: https://github.com/deis/tester/blob/master/master/master.cfg#L104

In the CI environment we are building each image with the same name as its subfolder and adding a build-specific tag, for example deis/controller:tester-11, then using that tag to run and test it. This could be trickier if we merge this PR.

[mboersma]

Looking at this again, it's just a change to the master build target, and each container already has its own make build, so the deis/tester is already making assumptions that could be broken. It will be deis/tester's responsibility to keep its strategy current regardless. See https://github.com/deis/tester/issues/17.
LGTM.

[carmstrong]

LGTM