* exec2: implement dynamic workload users taskrunner hook
This PR impelements a TR hook for allocating dynamic workload users from
a pool managed by the Nomad client. This adds a new task driver Capability,
DynamicWorkloadUsers - which a task driver must indicate in order to make
use of this feature.
The client config plumbing is coming in a followup PR - in the RFC we
realized having a client.users block would be nice to have, with some
additional unrelated options being moved from the deprecated client.options
config.
* learn to spell
