Add support for erasure pools in ceph rbd provisioner

[sagor999]

This adds support for ceph rbd provisioner to be able to create rbd images with erasure pool. In the storage class definition for your kubernetes cluster, you would specify: pool: dataPool:

Fixes this issue: https://github.com/kubernetes-incubator/external-storage/issues/1096

Tested on the local ceph cluster.

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[sagor999]

/assign @rootfs

[sagor999]

/assign @cofyc

[cofyc]

/lgtm /approve Thanks!

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cofyc

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Needs approval from an approver in each of these files: - ~~[ceph/rbd/OWNERS](https://github.com/kubernetes-incubator/external-storage/blob/master/ceph/rbd/OWNERS)~~ [cofyc] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment