checkbox-touch: add picking of test-plan

This patch makes user pick test plan if there is more than one.
It removes the command-line argument --testplan.

Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge of lp:~kissiel/checkbox/pick-test-plan/ by tarmac [r=zkrynicki][bug=][author=kissiel]"