[ML] Do not download the ml-cpp zip when building it locally (elastic/x-pack-elasticsearch#2262)
[ML] Do not download the ml-cpp zip when building it locally (elastic/x-pack-elasticsearch#2262)

When the machine-learning-cpp repo is built locally, the zip file it
creates is preferred over that downloaded from s3 when creating the
overall x-pack-elasticsearch zip.  However, prior to this change the
build would ALSO download an ml-cpp zip from s3, and just not use it.

Original commit: elastic/x-pack-elasticsearch@711f2e4997216703cf813ae9354b75bd1541b0ca

vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra