* I had to contextualise Communicator.Start and RemoteCmd.StartWithUi
NOTE: Communicator.Start starts a RemoteCmd but RemoteCmd.StartWithUi will run the cmd and wait for a return, so I renamed StartWithUi to RunWithUi so that the intent is clearer.
Ideally in the future RunWithUi will be named back to StartWithUi and the exit status or wait funcs of the command will allow to wait for a return. If you do so please read carrefully https://golang.org/pkg/os/exec/#Cmd.Stdout to avoid a deadlock
* cmd.ExitStatus to cmd.ExitStatus() is now blocking to avoid race conditions
* also had to simplify StartWithUi
* removed packer.Cache and references since packer.Cache is never used except in the download step. The download step now uses the new func packer.CachePath(targetPath) for this, the behavior is the same.
* removed download code from packer that was reimplemented into the go-getter library: progress bar, http download restart, checksuming from file, skip already downloaded files, symlinking, make a download cancellable by context.
* on windows if packer is running without symlinking rights and we are getting a local file, the file will be copied instead to avoid errors.
* added unit tests for step_download that are now CI tested on windows, mac & linux.
* files are now downloaded under cache dir `sha1(filename + "?checksum=" + checksum) + file_extension`
* since the output dir is based on the source url and the checksum, when the checksum fails, the file is auto deleted.
* a download file is protected and locked by a file lock,
* updated docs
* updated go modules and vendors
Per code review feedback, the Go template library supports
encoding variables in URL query format. Instead of exposing
two different public key formats (unmodified string and a
URL encoded string), just have the user apply the 'urlquery'
modifier to their template.
The SSH public key (in authorized_keys format) can be retreived in
the build template by specifying '{{ .SSHPublicKey }}'. The key is
URL encoded to escape spaces.
