Since now we have Python 3.5 for RHEL use it in Docker

[daniviga]

To be able to use Pyhton 3 in our Docker container with CentOS we were doing ugly things.

Now we can take the advantage of having our own Python 3.5 package and make code much, MUCH more clean and simpler.

https://ci.openquake.org/job/builders/job/docker-builder/103/