Dockerfile: Add ENV configuration

* Add ENV configuration options to Dockerfile as opposed to keeping in
  the wrapper script.
* First step to splitting up openvpn.sh in to smaller scripts.
openvpn.sh: Split in to smaller scripts

* Split soon to be massive wrapper into smaller managable scripts.
* Re-organized Dockerfile to exploit cache when rebuilding
ovpen_init: Remove external IP resolution

* Disable auto guessing the external IP in favor of the user explicitly
  specifying the server name.  Save the servername for client cert
  generation later.
* Remove dnsutils from build since dig is no longer necessary.  Favor
  learn and mean images.
Dockerfile: Switch to leaner Debian image

* Debian testing/Jessie is approximately 30% smaller the Ubuntu, use
  that instead.