User guide Ironic section improvement

[Pavel Vaylov]

In MOS user guide https://docs.mirantis.com/openstack/fuel/fuel-8.0/user-guide.html#ironic-configure
we have several things to fix\improve:

- Guide suggests to create images manually but there are no way to do it on MOS nodes.
To install disk-image-builer pip is required but there is non package for "pip" in repositories.

I suggest to add that users can use Ubuntu RAW (and only raw, qcow converted to raw doesn't work with Fuel Ironic drivers) cloud image taken from official source: https://cloud-images.ubuntu.com/trusty/current/trusty-server-cloudimg-amd64.tar.gz

- Properties for Glance image should be corrected:
What we have:
glance image-update <image-id> --property cpu_arch=x86_64
                               --property hypervisor_type="baremetal"
                               --property fuel_disk_info=DISK_INFO

"DISK_INFO" in this case is variable and in must be $DISK_INFO

Also DISK_INFO must be defined before updating of image in glance.

- Default disk schema has typo:
‘[{"name": "sda", "extra": [], "free_space": 11000, "type": "disk",
"id": "vda", "size": 11000, "volumes": [{"mount": "/", "type":
"partition", "file_system": "ext4", "size": 10000}]}]’

sda should be changed to vda as well as we should clarify that vda should be used in deploying on VM case.