[TEST] Add basic upgrade test script

This commit adds a basic full cluster restart upgrade script that tests
upgrades of elasticsearch across major version. In contrast to the BWC
tests we are running using the java test framework this test uses a python
script as well as the REST APIs to ensure upgrades across major version work and
indices are compatible.

The upgrade test starts 2 or more nodes of an old elasticserach version, indexes
a random number of documents into the running nodes and executes a full cluster restart.
After the nodes are recovered a small set of basic checks are executed to ensure all
documents are still searchable and field data can be loaded etc.