provisioners/shell: fix validation for args [GH-1949]

The logic change in 57d477514067a37665386968ef0a37abf86e3380 introduced
a bug where neither strings nor arrays provided as `args` for shell
provisioners would pass validation.

This fixes that problem along with a few extras:

- split out arg validation into a private method
- update comment describing valid args
- add a few unit tests around config validation
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 !/