Set u+w perms on private keys in tmp dir.

This should allow additional `vagrant provision` calls to work when
using the salt provisioner and providing private keys. See #3542.
Merge pull request #3575 from shadowfax-chc/salt-provision-private-key-perms

provisioner/salt: Set u+w perms on private keys in tmp dir.
Update CHANGELOG.md
website/www: feature preview docker provider
providers/docker: raise better error for local executor
providers/docker: clearer options on docker-logs