Update openSUSE installation instructions

Out of box, the 'sudo' command on openSUSE does not work similarly to other distros.  In this case, the path to usermod should be explicitly given, since /usr/sbin, where usermod is located, is not on the sudoing user's PATH.  The other sudo commands here should be fine.
Merge pull request #7634 from krrg/patch-1

Update openSUSE installation instructions