contint: provision hhvm on CI slaves
We need some hhvm configuration on the CI Trusty slaves so we can run
PHPUnit tests (T75521).
Create a new class contint::hhvm which invokes ::hhvm with settings
appropriate for CI.
Do not use a local repo and only rely on central one. Its path is made
empty to have hhvm fallback to HHVM_REPO_CENTRAL_PATH env variable. It
is set in Jenkins global configuration to vary per build using