add support for incremental Gradle builds

[vorburger]

Once the base #118 (#121) is in, I'll follow-up on it to add support for --incremental Gradle builds.

I'll have to find a way to make Gradle write everything we want to preserve into ${S2I_ARTIFACTS_DIR}/gradle, which I've already used in the final clean-up commit of #121 for GRADLE_CLEAR_REPO.

We'll then also have to make java/templates/s2i/save-artifacts include that in the TAR (how, together with m2?), and I'm guessing somewhere "restore" it - @rhuss where does that happen?

I can then test this with the new test.sh (just manually unfortunately; I don't see how to reasonably easily automatically cover this...) and observe that the 2nd incremental Gradle run is not re-downloading anymore, and that the 2nd Maven incremental run still not re-downloading (as it currently, just to verify non-regression).

[vorburger]

find a way to make Gradle write everything we want to preserve into ...

in #118 I had envisioned this to be:

--gradle-user-home=${S2I_ARTIFACTS_DIR}/gradle-home 
--project-cache-dir=${S2I_ARTIFACTS_DIR}/gradle-project-cache

but based on above I'll probably instead make it:

--gradle-user-home=${S2I_ARTIFACTS_DIR}/gradle/user-home 
--project-cache-dir=${S2I_ARTIFACTS_DIR}/gradle/project-cache

#118 also mentions:

BTW incremental builds with Gradle should be blazingly fast, not only avoiding dependency re-downloads, but truly incrementallying building code changes themselves

but I now realized that would require to save-artifacts and restore of ALL build/ (sub?!?) directories... that's likely more complicated than I could be bothered for at this stage, and so is not a goal of this #118 follow-up issue, anymore. Saving the re-download of depenencies from external repositories is a great first step, already.

[rhuss]

@vorburger hey, just a quick note: I'm at Red Hat and jump onto the s2i images at the beginning of next week. sorry for the delay ...