[docs] Change title of 'Confirm hardware' section in the user guide

[Eugene Korekin]

I propose to change a title of 'Confirm Hardware' section (https://docs.mirantis.com/openstack/fuel/fuel-master/user-guide.html#confirm-hardware) to something like "Underlying infrastructure requirements".

The reasons for this:

1) This section already have some non hardware-specific requirements, for example requirement regarding DHCP server.
2) We want to add some requrements to this section that aren't related to hardware, for example see https://bugs.launchpad.net/fuel/+bug/1526452