Allow data-root to be an absolute path

[munnerz]

Currently, because the data root field on the filesystem/storage struct is plumbed through the new Golang FS interface, it does not support absolute paths.

This makes it confusing for users when configuring their CSI driver deployments, as a relative path must be used (meaning they must have knowledge of the working directory configured in the container).

At the moment, because the working directory is /, this means that users have to strip the leading / from the data directory argument when starting their CSI driver.

/kind bug
/priority important-soon

[irbekrm]

It would be good to have some links to which parts of the code this issue refers to.