Validate download_root option

Without this commit, there is no validation on the download_root option.
This option helps the archive manager know where to find the installer
tarballs for available PE versions.

This commit validates that the download_root value is either
UNSET_VALUE, a valid file path, or a valide uri.
Archive prints available installers on error

Without this commit, the archiver does not try to fail when no method of
finding an installer is encountered. Instead it simple just waits to
fail on the download attempt.

This commit changes the archiver to detect when an installer is not
present and also when no download_root is available. The archiver will
now respond with a message letting the user know no installer of the
desired version is available and display the list of available
installers.
Add I18n Support

Without this commit, there is no I18n support in pe-build. I18n is used
in various places in Vagrangt including exceptions classes. Without this
support, a new exception class would require overriding a fair amount of
functionality in the `Vagrant::Errors::VagrantError` class. There are
also various other places this support will be beneficial as well.

This commit adds I18n support to this plugin. This is achieved by adding
a load statement and a basic default translation file for the english
language.
Use exception to fail in PEBuild::Archive

Without this commit, the `PEBuild::Archive` class would fail out of
execution using the `Vagrant::Errors::VagrantError` exception. This is
not very specific or handleable for other systems relying on or working
with the PEBuild plugin.

This commit adds a `PEBuild::ArchiveNoInstallerSource` exception class
and the appropriate message to the english translation file for the I18n
component of Vagrant execption classes.
`download_root` defaults to `nil`

This commit partially undoes commits 0fe10c0c4d and 77b5d4c860 which
changed the default for `download_root` to
`Vagrant::Plugin::V2::UNSET_VALUE`. This commit restores the default
value to `nil` as that is safer to be passing around in the long term.
Minor change to inline documentation
Extract pe-build config merging logic
Rename late_config_merge, ensure configs are finalized
Merge branch 'late_finalize_config'
Add comments about finalizing in pe_bootstrap provisioner
Merge pull request #15 from trlinkin/download_root_enhancement

Download root enhancements