normalize env var handling
normalize env var handling

- have all env vars start with BEAKER_
- process all env vars in one hash
- set all default values in one hash

puppetlabs/beaker-rspec,mcanevet/beaker-rspec,rooprob/beaker-rspec,anodelman/beaker-rspec,petems/beaker-rspec-windows,electrical/beaker-rspec