core: Fix regression caused by recent changes that allowed multiple private keys [GH-907]
core: Fix broken SSHExec spec
Merge pull request #2557 from fgrehm/fix-vagrant-ssh

core: `vagrant ssh` and `vagrant ssh -c` should handle multiple SSH keys
Merge pull request #2556 from phinze/fix-shell-array-args-validation

provisioners/shell: fix validation for args [GH-1949]
provisioners/shell: s/unless/if !/
[providers/virtualbox] support DHCP interfaces for NFS