Use monochrome output for cucumber
Factor out NodeLabelParameter plugin out of Build class
Fix NodeLabelParameterPluginTest#run_on_several_online_and_offline_slaves instead of ignoring
[JENKINS-23069] A bit nicer way of sanity checking

There are several problems:

1) Checking after navigation is not enough as it is triggered only if navigating explicitly via WebDriver. Clicking links/buttons is not covered.
2) Sanity checking need to access page source witch triggers UnhandledAlertException in case alert is present, in firefox alert is dismissed at this point.
3) Checking if alert is present upfront (`driver.switchTo().alert()`) is hideously slow (1-2 seconds).

Moving the check to #beforeClickOn and #beforeNavigateTo so it is no longer necessary to handle alerts (any alert present at this point is clearly unhandled). We can add checking to #onException at any time.

This fixes DiskUsagePluginTest#reflect_disk_changes_in_disk_usage_report (problem #2).