nova reserves space for the instance

[Michael Kraynov]

Steps for reproduce:

1. Ceph/iSCSI for volumes.
nova hypervisor-stats
| local_gb | 29 |
| local_gb_used | 0 |

2. Launch an instance "Boot from image(creates new volume)"

Flavor: m1.small (root disk 20Gb)
Create volume : 25Gb

nova hypervisor-stats

| local_gb | 29 |
| local_gb_used | 20 |

But we don't have any disk on the compute node:
ls -lh /var/lib/nova/instances/f463e3a3-ef8c-42fb-b719-3913203d3d2d/
total 24K
-rw-rw---- 1 root root 19K Nov 10 15:09 console.log
-rw-r--r-- 1 nova nova 1.7K Nov 10 15:08 libvirt.xml

cinder list
+--------------------------------------+--------+--------------+------+-------------+----------+--------------------------------------+
| ID | Status | Display Name | Size | Volume Type | Bootable | Attached to |
+--------------------------------------+--------+--------------+------+-------------+----------+--------------------------------------+
| 374f40a8-967b-44f7-831a-47ada3aa2db9 | in-use | | 25 | None | true | f463e3a3-ef8c-42fb-b719-3913203d3d2d |
+--------------------------------------+--------+--------------+------+-------------+----------+--------------------------------------+

Nova reserves space which actually is not using.