OpenShift install docs: Recommend "add-scc-to-user" over "edit scc"

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Minor documentation change to Openshift Installation - Security Context Constraints.

Recommend using oadm policy add-scc-to-user to anyone using OpenShift v3.1 and above (the majority of people now).

This is much less error prone and easier to script than manually editing or patching the SCC's with oc edit scc. add-scc-to-user was introduced in v3.1. (Unfortunately much third party documentation and comments still suggest manual editing).

https://docs.openshift.com/enterprise/3.1/admin_guide/manage_scc.html#grant-access-to-the-privileged-scc