Allow plans to _require_ global or plan variables to be defined

Currently, for our projects we have two plans for each: a snapshot plan which detects SVN changes and builds automatically and a release plan which invokes the maven release process. The maven release plan uses two plan variables to indicate the "current version" being released and the "next" version which are then fed into the release:prepare goal for maven to use. This works wonderfully.

As a minor improvement it would be nice to require the user to run a parameterized plan, specifying the values for these plan variables. For this kind of plan, it is invalid to run automatically – it must require new values to be input whenever the plan is run. Thus, it would be nice to have first class support from bamboo to prevent someone from running the automated plan accidentally with blank values for the variables.

The risk impact of running a plan with blank variables is very low as the maven:release plugin validates these early in the build process, but this is kind of a nice-to-have feature of Bamboo I think.