If you run `./gradlew -p qa bwcTest -Dtests.distribution=zip` then we
need to resolve older versions of the default distribution. Since those
aren't available in maven central, we need add the elastic maven repo to
the project.
This configures all `qa` projects to use the distribution contained in
the `tests.distribution` system property if it is set. The goal is to
create a simple way to run tests against the default distribution which
has x-pack basic features enabled while not forcing these tests on all
contributors. You run these tests by doing something like:
```
./gradlew -p qa -Dtests.distribution=zip check
```
or
```
./gradlew -p qa -Dtests.distribution=zip bwcTest
```
x-pack basic *shouldn't* get in the way of any of these tests but
nothing is ever perfect so this we have to disable a few when running
with the zip distribution.
This commit makes x-pack a module and adds it to the default
distrubtion. It also creates distributions for zip, tar, deb and rpm
which contain only oss code.
This commit lessens the burden on configuring settings.gradle when new
projects are added. In particular, this makes it trivial to move a
plugin to a module (or vice versa).
Nik Everett
Ryan Ernst