Removes default admin/admin credentials from global admin.conf

This removes the admin/admin credentials from being used as defaults. It also updates the documentation so that users know username/password can be put into ~/.pulp/admin.conf

I worked on a validation that disallows passwords from being added to the global admin.conf, but the run_test.py was adding them in during the test runner. That was an unexpected side effect and isn't the real point of this PR so I backed out of that change. At a minimum the example was removed from the global admin.conf.