Fix Gradle >=4.2 compatibility

[ywelsch]

Gradle 4.2 introduced a feature for safer handling of stale output files. Unfortunately, due to the way some of our tasks are written, this broke execution of our REST tests tasks. The reason for this is that the extract task (which extracts the ES distribution) would clean its output directory, and thereby also remove the empty cwd subdirectory which was created by the clean task, see Gradle logging output:

:modules:lang-painless:integTestCluster#extract
Putting task artifact state for task ':modules:lang-painless:integTestCluster#extract' into context took 0.0 secs.
Deleting stale output file: /home/ywelsch/dev/elasticsearch/modules/lang-painless/build/cluster/integTestCluster node0
Up-to-date check for task ':modules:lang-painless:integTestCluster#extract' took 0.0 secs. It is not up-to-date because:
  No history is available.
:modules:lang-painless:integTestCluster#extract (Thread[Task worker for ':' Thread 8,5,main]) completed. Took 0.351 secs.

Subsequent tasks (installing a plugin or running the node) would then fail because of the missing cwd directory.

The reason why Gradle would remove the directory is that the earlier running clean task would programmatically create the empty cwd directory, but not make Gradle aware of this fact, which would result in Gradle believing that it could just safely clean the directory.

This PR explicitly defines a task to create the cwd subdirectory, and marks the directory as output of the task, so that the subsequent extract task won't eagerly clean it. It thereby restores full compatibility of the build with Gradle 4.2 and above (tested on my CI).

In the future, we should revisit the way we write these tasks, there are way too many tasks that copy stuff into common output folders, which also hurts making the build incremental (see The case against overlapping outputs).

[rjernst]

LGTM, thanks @ywelsch!

[jasontedor]

LGTM but I am testing this on my CI first where I repeatedly run into this problem.

[ywelsch]

I've done more testing on this, which uncovered two smaller bugs:

• I had wrongly defined the task dependency of the new task, fixed in 5e98e4b. This was uncovered by running gradle check without clean.
• While the build worked with Gradle 4.2, there would still be an unrelated problem with Gradle 4.3. Gradle 4.3 does not seem to like the @Input annotation on the waitCondition closure of AntFixture anymore, as it now does some extended input/output validation. I've simply removed the annotation in a9a7abc.
  With these two fixes in place, I've run the build successfully with both Gradle 4.2 and 4.3.