CB-6899 Fixes 'Could not adapt' exception while parsing master.cfg

Fixes `error while parsing config file: ('Could not adapt', (<buildbot.steps.shell.ShellCommand object at 0x5415750>,), <InterfaceClass buildbot.interfaces.IBuildStepFactory>) (traceback in logfile)` error while parsing master.cfg introduced in commit https://github.com/MSOpenTech/cordova-medic/commit/628926883ba59fc941c696628276b55b9e590385