1804 config file tool

* New paster command "config-tool" for editing a CKAN config file
* Dockerfile uses it

Issue: https://github.com/ckan/ckan/issues/1804
Discussion of this solution and alternatives: https://github.com/ckan/ckan/pull/1828

I've only partially tested the Dockerfile bit - the output from the start-up looks fine but I've not got as far as running the build ok. So it would be great if someone can confirm a build runs ok.

The only other thing I'm keen to change is in install-using-docker.rst - it says on line 138 when customizing Docker to "Add my custom configuration file" and I really think it would be far better to encourage people to use the "config-tool" to edit the ini instead. However I'm not clear how that would be done in a Dockerfile, since the ini is not created until runit runs. @nickstenning can you help here?