(QENG-1054) Add functionality to install_pe to modify answers...

...file before install

- add ability to add custom_answers to individual hosts to be appended
  to answer file
- rework answer file code so that answers are an object that can be
  queried for an answer string
- instead of querying env for individual answers just pull in anything
  that matches q_*
- pull the default answer values into the presets so that it is more
  easily tracked
Merge pull request #464 from anodelman/answers

(QENG-1054) Add functionality to install_pe to modify answers...