Clarify values of hypervisor disk attributes

[Andreas Scheuring]

At the moment dummy values are returned on

openstack hypervisor show host2

| disk_available_least | 1024
| free_disk_gb | 1024
| local_gb | 1024
| local_gb_used | 0

Clarify which values make sense!

Same for

openstack host show host2
+-------+----------------------------------+-----+-----------+---------+
| Host | Project | CPU | Memory MB | Disk GB |
+-------+----------------------------------+-----+-----------+---------+
| host2 | (total) | 3 | 55555 | 1024 |
| host2 | (used_now) | 1 | 1024 | 0 |
| host2 | (used_max) | 1 | 1024 | 0 |
| host2 | 0e7b76a92ad54aa0be6b0a61c391db05 | 1 | 1024 | 0 |
+-------+----------------------------------+-----+-----------+---------+