Conditionals in migrations should be able to call tools

Currently the first "argument" in an conditional in a migration must be a boolean value. (Well it doesn't has to but it will be cast to a boolean)

Some people (even ourselves) however, confuse this with the possibility to make a call to one of the tools. Like:

array('if', array(

    array('module.exists', ...),

    array('module.remove', ...),

),

This will currently always be true since it's a non-empty array.