Add support for multiple instances (several websites) to Ubuntu (deb) packages

My organization is planning to host 2 CKAN instances on the same server. So far, the only server we have with several CKAN instances has CKAN installed from source, however we would prefer to use a managed CKAN for new servers.

Unfortunately, while CKAN itself does support running any number of instances from a single code copy (according to @wardi ), the Debian packages do not support hosting several CKAN instances. At least, that's what [the maintainer's guide has been claiming](https://docs.ckan.org/en/2.9/maintaining/installing/index.html#installing-ckan) for at least 7 years:
````
You should install CKAN from package if:
[...]
You only want to run one CKAN website per server
````

I have no idea what specific issues prevent that, but identifying them in this ticket would be great. [An old mailing list post](https://lists-archive.okfn.org/pipermail/ckan-dev/2013-October/017421.html) lists the elements to consider to host several CKAN instances on a single OS.

Start with the package-installation section of the CKAN maintainer's guide and the linked 2013 mailing-list post, which lists elements relevant to multiple instances. Identify the package-specific blockers and document or implement the changes needed to support several CKAN websites on one server. Done means the package limitation is removed and the supported setup is described.

Written by the indexing model from the issue text.