Remove un-needed context

  * BUILD_PATH is no longer used
Clean up contexts and make specs more explicit
Push ArchiveFilename down into StemcellBuilderOptions
StemcellBuilderCommand sanitizes the stemcell environment
Refactor StemcellBuilder to reduce instances vars
StemcellBuilderCommand create StemcellEnvironment
Refactor initializer
Move StemcellBuilderOptions into Command class
Rename for clarity
Update method signatures for consistency

  * rake task now passes Build.candidate explicitly