Add support for a bundle/chain of ca's when configuring SSL.

In my case, I had four CA's in my certificate chain.  The method of
concatenating them together described in the documentation for awsbox
did not work in this scenario.

This pull request adds support for loading multiple CA certs
concatenated into one file (bundle.ca).  If the file does not exist,
processing continues as normal.

References:

- https://gist.github.com/pleaseshutup/3866484
-
https://github.com/nodejitsu/node-http-proxy/commit/10f6b0577518bdfcb6b4
3c1f516dc988bdcade53