Check for PUBLISH_READONLY capability before setting readOnly field in ControllerPublishVolume

[msau42]

See https://github.com/container-storage-interface/spec/pull/310/files for details

[jsafrane]

What should be the right behavior here? Fail attach() and report error when SP does not support PUBLISH_READONLY? Or silently fall back to read-write attachment and expect that kubelet mounts it as read-only?

[msau42]

I think it should not set read-only if the plugin doesn't support it. This is not backwards compatible though.

[msau42]

If we make the change before cutting an official 1.0 maybe that's fine