allow using ceph secret

[rootfs]

https://github.com/yp-engineering/rbd-docker-plugin/blob/master/driver.go#L815 assumes keyrings can be found at /etc/ceph/. This is sometimes impractical in a large scale setup where syncing /etc/ceph is not possible.

rbd can also take a secret, rather than looking up at /etc/ceph. An example can be found at https://github.com/kubernetes/kubernetes/blob/master/pkg/volume/rbd/rbd_util.go#L231

[hekaldama]

Good idea. Will look at this and how we can all collaborate instead of making our own rbd talkers. Will continue to push on contiv-experimental/volplugin#10.

[ps-account]

I think it would also be nice to be able to do this during the "docker run" invocation. But that could be a separate enhancement.