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

- make is possible to use arbitrary role names for most beaker DSL APIs
(QENG-431) expanded and improved role support in beaker (part 2)

- add support for running commands against hosts by name
- add support for arbitrary role methods, like those for built in roles
    eg, on master, "echo hello"
        on myrole, "echo hello"
Merge pull request #237 from anodelman/role-repair

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