Sometimes, ifdown is not enough.
provisioners/ansible: use the first private key

/cc @gildegoma - Does Ansible support multiple private keys? Vagrant
now does, so this isn't exactly safe.
Merge pull request #2539 from bearstech/debian_ifdown_ifup

guests/debian: ifdown is not enough for network config
website/docs: fix letter spacing on high zoom [GH-2486]
guests/redhat: DHCP_HOSTNAME set to hostname [GH-2441]
website/docs: better Chef docs [GH-2436]
website/docs: fix 404 box link
website/docs: better document creating base boxes
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