Pillar: implement *_source as an alternative to *_content

[alxwr]

There's no need to funnel data through ca_content, ta_content and others. This just slows down Pillar.

In my Pillar definitions I have do to stunts like this which are just not needed if a source file is provided:

       ta_content: |
         {{ __salt__['cp.get_file_str']('/path/to/predefined/openvpn_tls_auth/'+name+'.ta.key')|indent(8) }}