Fail if autidisk_size is less than 10GB

The disk size must be at least 10GB for ceph to work smoothly.
The reasons for minimum 10GB are:

* Ceph OSD journal need at least 2GB, otherwise journal creation fail
* OSD need at least 7-8GB to hold default and pools for openstack,
    otherwise data write to the pools will fail.
Merge pull request #81 from hkumarmk/ensure_minimum_ceph_autodisksize

Fail if autidisk_size is less than 10GB