Allows use of environment var for specifying config file.

Implements #1597

If you don't specify a config file using the -c option, CKAN defaults to
looking for development.ini in the current directory.  This patch will
first check the -c parameter, and if it doesn't find it will use the content
of the CKAN_INI environment variable.  If it cannot find that, it will then
default to development.ini in the current directory.

This does not work with paster serve as this is handled by pastescript which
specifically requires the -c paramter.