Add failsafe-style "testFailureIgnore" to nuxeo-distribution-tools

One of the functionalities of nuxeo-distribution-tools is to call ant tasks from maven.
However, in maven, when called in the "integration-test" phase, any failure is immediate, which means we can't use the intended lifecycle:

• pre-integration-test: setup things
• integration-test: run tests
• post-integration-test: tear down
• verify: check results

That's one of the things the maven-failsafe-plugin does, but when using nuxeo-distribution-tools instead we lose that functionality.
-> We need to add this failsafe functionality to nuxeo-distribution-tools so we can use a correct setup/test/teardown flow when running tests instead of putting everyting in the "test" phase.