tls: support reading multiple cas from one input

Before this commit you had to pass multiple CA certificates as an array
of strings.  For convenience you can now pass them as a single string.

Fixes: https://github.com/nodejs/node/issues/4096
PR-URL: https://github.com/nodejs/node/pull/4099


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: James M Snell <jasnell@gmail.com>