CloudMachineNamer not include underscore

- an underscore in the yaml app name caused failure to create machine
PR #1431 CloudMachineNamer: incorporate comments

- uses guava CharMatcher instead of regex
- this now converts “br-rc-“ to “br-rc”, whereas previously that was
  left as “br-rc-”
Merge pull request #1431 from aledsage/minor/machineName-not-include-underscore

CloudMachineNamer not include underscore