debian_build: fix variable usage and usage of check_call-method

debian_build: fix variable usage and usage of check_call-method

we shall stay with DISTRIBUTION envionment variable yet: let's force the user to
provide the necessary distribution in Jenkins GUI or in call_jenkins.
in the next step we try to remove this parameter from jenkins and base the
decision of what distribution to use on what we find in the changelog. question
to be answered for this:
 stable -> squeeze?
 our old values like 'production', pau, ppu, ... -> ??
 wheezy -> wheezy?

the check_call() method from subprocess seems to return the exit code of command
ran as int. this doesn't behave well in current if-syntax, so we changed it to
match for success code "0" explicitely.