Add block-based subvol type

[JohnStrunk]

Gluster-fuse (upon which gluster-subvol is based) struggles with metadata-intensive and small-file workloads.

We would like to have a subvol type that in analogous to gluster-block. It needs to preserve the original objectives of gluster-subvol (i.e., large PV count, thin provisioning to allow overcommit, etc.)

[ShyamsundarR]

Work list:

• Write a creator, akin to existing creator, that creates files as XFS formatted devices, and generates the required PV list for the same
• Write a gluster-block-subvol flexvol provider, that does the plumbing when a PVC for one of the above PVs comes to the client system
• Add a recycler to reclaim PVs that are in the released state
• Update all READMEs in the repo for the above created workflow
• Create a storage class yml for gluster-block-subvol
• Write ansible to deploy the recycler pod to a k8 or Openshift setup
• Test end-to-end deployment and use
• Investigate DIO in loop devices (currently the default is DIO, but not being honored)
• Check --nooverlap option for prevention of double mounts of the same image
• Add and test selinuxRelabel: true as a capability in the flex vol init call
• Add safety to ensure only pne consumer even in cases of faults is accessing the block device
• Check CentOS7 support for loop devices and any gotchas therein!