The MVP for CSI in the 0.11.0 release of Nomad did not include support
for opaque volume parameters or volume context. This changeset adds
support for both.

This also moves args for ControllerValidateCapabilities into a struct.
The CSI plugin `ControllerValidateCapabilities` struct that we turn
into a CSI RPC is accumulating arguments, so moving it into a request
struct will reduce the churn of this internal API, make the plugin
code more readable, and make this method consistent with the other
plugin methods in that package.