post-processor/vagrant: fix output ConfigTemplate validation [GH-324]

-apply same output validation steps found in builder specific Configure
functions
post-processor/vagrant/virtualbox: support ova artifacts [GH-315]
provisioner/shell: retry the upload if command fails as well

This improves reboot handling robustness
communicator/ssh: more error handlingin Upload
packer/rpc: less panics
packer/rpc: error instead of panic cancelling builder
packer/plugin: swallowInterrupts in plugin should swallow all
Update CHANGELOG
packer/rpc: don't panic on failed Ui calls
Merge pull request #331 from jasonberanek/vbox-vagrant-ova

post-processor/vagrant: support for Virtualbox OVA artifacts
Update CHANGELOG
post-processor/vagrant: nitpick format
Merge pull request #329 from jasonberanek/vagrant-output-fix

post-processor/vagrant: proper output ConfigTemplate validation [GH-324]
packer: no need to check if nil since we're appending to slice
post-processor/vagrant: don't error if unused
Update CHANGELOG
Fix #264 to upload local minion config

Conflicts:
	provisioner/salt-masterless/provisioner.go
	website/source/docs/provisioners/salt.html.markdown
website: delete salt.html.markdown
Update CHANGELOG
builder/amazon/instance: remove check for AMI tools [GH-330]