copy action plug-in check mode respects force=no

The copy action accepts force=no, which tells it not to replace an
existing file even if it differs from the source.  The copy action
plug-in wasn't respecting this option when operated in check mode, so it
would report that changes are necessary in check mode even though copy
would make no changes when run normally.

Runner._remote_md5 was changed to make the logic for setting rc perhaps
a little more clear, and to make sure that rc=0 when the file does not
exist.