When running in check mode, some prechecks previously failed because
they use the command module which is silently not run in check mode.
Other prechecks were not running correctly in check mode due to e.g.
looking for a string in empty command output or not querying which
containers are running.

This change fixes these issues.

Closes-Bug: #2002657
Change-Id: I5219cb42c48d5444943a2d48106dc338aa08fa7c