[docs] Add a clarification regarding necessity of a public gateway

[Eugene Korekin]

It's not obvious from our documentation that a working public gateway is a must for a successful cluster deployment.
We often face partners which deploy their clusters without specifying a correct gateway in Fuel network settings.

I think this requirement should be stated explicitly in our documentation, but right now I cannot find such statement in it.
I propose to add this requirement to the section "Confirm hardware" https://docs.mirantis.com/openstack/fuel/fuel-master/user-guide.html#confirm-hardware. While it's not strictyl hardware related, this section already have non-hardware requirements.

I also propose to change the title of the section, but I'll create a separate bugreport for this.