website/docs: clarify when provisioners are run [GH-2541]
website/docs: document synced folder options [GH-2274]
provisioners/chef: validate environments path [GH-2381]
core: output available commands in machine-readable
core: fix tests
commands/box/add: custom CA cert [GH-2337]
core: can specify config.vm.box_download_ca_cert
core: validate box_download_ca_cert_file, expand