Remove creating admin endpoints from install-guide

[Dr. Jens Harbott]

Currently the instructions create public+internal+admin endpoints for all services.

The admin endpoint is useful only for Keystone, so it should be dropped for all other services.

The internal endpoint in the current setup is always the same as the public endpoint and thus could also be dropped.

[Lana (loquacity)]

Since our base Install Guide includes Keystone, I would argue that including the admin endpoint (and keeping the instructions consistent across chapters) is worthwhile. Happy to argue the point, though.

[Dr. Jens Harbott (j-harbott)]

1. Even if you install an admin endpoint for Keystone, that does not imply that you also need to create an admin endpoint for every other service.

2. Thinking once more, with Identity API v3, even Keystone doesn't need an admin endpoint anymore, the functionality of the pipeline is the same as for the other endpoints. So we could be dropping admin endpoints completely and still be consistent.