plainbox:commands:run: fix non-interactive run

This patch fixes plainbox run --non-interactive that would previously
crash with:

    UnboundLocalError: local variable 'job_result' referenced before assignment

Fixes: https://bugs.launchpad.net/plainbox/+bug/1399685

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/fix-1399685/ by tarmac [r=kissiel][bug=1399685][author=zkrynicki]"