Update Jenkins scripts to retrieve test parameters

Update `jenkins.sh` and `jenkins_integration_tests.sh` so that it
retrieves test parameters used for integration tests from a git
repository in Github Entreprise.

This allows us to access the test parameters using the vCloud Tools
Tester gem, which is supported as of
e03b40b50d0ac1e854024e1c00f6e8dcd675eea3.
Merge pull request #73 from alphagov/71835572-update-jenkins

Update Jenkins job to `git clone` test parameters