adds options for client certs to downloader class

Adds :client_cert to the env array and then passes it into the curl
options array
adds param for client certs

adds an option to the command parser for client certicates
adds client cert
adds client cert to default config
adds accessor for client_cert
passes client cert option from Vagrant file to box_add
Merge branch 'master' into client-certs
provisioners/puppet: update config to new style
provisioners/puppet: can specify a client key/cert
provisioners/puppet: make the output a bit clearer
provisioners/puppet: use better flags for one-time run
core: allow multiple private keys [GH-907]
provisioners/chef: upload encrypted data bag secret to prov path
[GH-1246]
Merge branch 'client-certs' of https://github.com/sthulb/vagrant into sthulb-client-certs

Conflicts:
	config/default.rb
	lib/vagrant/action/builtin/handle_box_url.rb
	plugins/commands/box/command/add.rb
kernel/v2: properly set default value for client cert
Update CHANGELOG
website/docs: document client cert
kernel/v2: new style default box url
core: cleanup HandleBoxURL
core: config.vm.box_url can be array of urls [GH-1958]