Description of problem:

When scheduling a set of tasks based on a related package it is
essential to be able to select non-destructive tasks only. From
the Beaker source it seems currently only the following values are
accepted:

    The *filter* argument must be an XML-RPC structure (dict),
    with any of the following keys:

        'install_name'
        'osmajor'
        'names'
        'packages'
        'types'
        'valid'

Please, implement "destructive" field as well. Thanks.

Version-Release number of selected component (if applicable):
0.6.15

Steps to Reproduce:
self.workflow.hub.tasks.filter(
        dict(packages=["wget"], valid=True, destructive=False))