The core REST tests with security currently use a hardcoded username and
password. This is not amenable to running these tests in scenarios where
the user controls the creation of the cluster and owns the credentials
for this cluster. This commit enables running the core REST tests with
security with a custom username and password.
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.
