installation: fix variable name & missing module

* Fixes wrong variable name causing demosite population to fail.

* Adds missing module qrcode to requirements.

* Fixes error output during demosite population.
config: add site configuration loading

* Adds loading of default site specific configuration from
  entry points.
Merge pull request #190 from lnielsen-cern/pu-fixes

Minor fixes + default site configuration
scripts: data file option for demosite populate

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>