This will allow us to use its content from other languages than Lua as
well.

check_site.sh and site.sh were setting GLUON_SITE_CONFIG on their own
even though the environment variable was already exported.

Let the site configuration value script return 1 when
the requested value is not available.This enables us
to react on failures, so we can for example use a default
value.

With this patch the script will return with exitcode 1 if
the value is not found and no output on stdout. Otherwise
exitcode is 0 and the value is returned on stdout.