Cleanup provisioner 'ui' usage

Without this commit, "ui" access is somewhat indirect in the
`PEBuild::Provisioner::PEBootstrap` class.

This commit simply cleans up the access to "ui" resource for the
"machine" context in  PEBuild::Provisioner::PEBootstrap class. This
class is accessing the "ui" resource by `@machine.env.ui` while the
`Vagrant::Machine` class supports more direct access via
`Vagrant::Machine#ui`.
Merge pull request #9 from trlinkin/ui_cleanup

Cleanup provisioner 'ui' usage
Clean up defaults
Merge branch 'set_defaults'
Whitespace fixup
Add commented out autosign
Error on Archive#unpack_to invocation
Extract file copy logic from archive
Add explicit rubygems source to Gemfile
Add minitar as a gem dependency
Implement naive tar unpacker.
Use correct variables when downloading archive
Remove old PE build actions
Bugfixes to unpacking archive.
Merge branch 'refactor_archive_copy'
Merge branch 'refactor_archive_unpack'
Move global config opts into pe_bootstrap config
Deprecate use of global pe_build config
Merge branch 'deprecate_global_config'
Set download variables as UNSET on initialization