Revert "Revert "(QENG-431) expanded and improved role support in beaker""
(QENG-431) (cleanup) expanded and improved role support in beaker

- cleanup in response to review comments
(QENG-431) (cleanup + split out filtering) expanded and improved role support in beaker

- split out filtering from applying the block to hosts
- block_on now determines if we have a filter or not
- run_block_on accepts both a set of hosts and an optional filter
Merge pull request #362 from anodelman/revert-343-revert-237-role-repair

(QENG-431) expanded and improved role support in beaker