cinder-volume: prevent LVM backend overallocation when creating snapshots

[vu tran]

When cinder-volume LVM backend is configured as thin-provisioning, when requesting a snapshot there is currently no check for free space. This allows us to create a large volume, dirty it, make a snapshot, rewrite the original volume, and run out of space in the thin pool.

To reproduce on devstack:

* On controller node, modify /etc/cinder/cinder.conf to have "max_over_subscription_ratio = 1.0" and "lvm_type = thin" and restart cinder-volume service.
* On my devstack, the default thin-pool stack-volumes-lvmdriver-1 has size of 9.51g
* Create 6g cinder volume
* Spawn VM that has volume created above attached
* Log into VM and dirty all /dev/vdb with dd
* On Controller node, create a snapshot of the volume
* Log into VM and dirty all /dev/vdb with dd again
* The thin pool stack-volumes-lvmdriver-1 reaches 100% data usage

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/348491

[OpenStack Infra (hudson-openstack)]

Change abandoned by Sean McGinnis (<email address hidden>) on branch: master
Review: https://review.openstack.org/348491
Reason: This review is > 4 weeks without comment and currently blocked by a core reviewer with a -2. We are abandoning this for now. Feel free to reactivate the review by pressing the restore button and contacting the reviewer with the -2 on this review to ensure you address their concerns.

[Sean McGinnis (sean-mcginnis)]

Unassigning due to no activity for > 6 months.