nuxeo-war-tests must be compliant with nuxeo.home and zip.file parameters

The current implementation of the custom prepare-environment target (https://github.com/nuxeo/nuxeo/blob/master/nuxeo-distribution/nuxeo-war-tests/itests.xml#L15-L27) is not compliant with the customization of the tested environment.
Here the need was to run those tests against the datebase release. See https://qa.nuxeo.org/jenkins/job/Deploy/job/IT-nuxeo-master-tests-cap-tomcat/
This prevent from testing the WAR pack feature on a release candidate (or any custom server).
We must be able to provide NUXEO_HOME, nuxeo.home or zip.file parameter.

The issue likely resides in the current prepare-environment target which maybe simply needs to redispatch the work between prepare-tomcat and download tasks...

See https://github.com/nuxeo/tools-nuxeo-ftest/blob/master/README.md