Use Zuul cloner to test mediawiki/core + vendor

Use the python cloning script I wrote for Zuul which clones multiple
repositories and apply the relevant Zuul ref / branch.

Being used for cross testing between:

    mediawiki/core
    mediawiki/vendor

Runs the whole PHPUnit test suite.

Generates:
    mediawiki-vendor-integration

Change-Id: Ibbf742e4aea247cfa5265c65e679a905b26517c8