Cover openSUSE and SUSE Linux Enterprise on the same page

Updated the documentation to cover the installation of Docker on
openSUSE and on SUSE Linux Enterprise.

Docker-DCO-1.1-Signed-off-by: Flavio Castelli <fcastelli@suse.com> (github: flavio)
Merge pull request #9194 from flavio/update_installation_docs

Update installation docs