Fix issue in Recover Task.

[shishir-a412ed]

This PR fixes Issue # 3 described in this comment

Hashicorp/nomad issue: hashicorp/nomad#9750

[shishir-a412ed]

@shivdudhani Can you please review it?

1. This PR fixes the error in decoding driver config

Dec 17 18:47:14 ip-10-102-98-114 nomad[27030]:  client.driver_mgr.containerd-driver: HELLO: RecoverTask: Failed to decode driver config: driver=containerd-driver @module=containerd-driver timestamp=2020-12-17T18:47:14.167Z
Dec 17 18:47:14 ip-10-102-98-114 nomad[27030]: client.alloc_runner.task_runner: error recovering task; cleaning up: alloc_id=2d6b365f-cc89-b230-2af9-a37ccbf0f6c8 task=adaas-task error="rpc error: code = Unknown desc = failed to decode driver config: EOF" task_id=2d6b365f-cc89-b230-2af9-a37ccbf0f6c8/adaas-task/43e03bbd

I have removed the logic for decoding driver config from RecoverTask since it's not needed at all.
For recovering the task, we get the container name from the task handle supplied by the nomad client during the RecoverTask API call. That container name is sufficient to get the container, get the task, check the status of the task and reattach to the existing task.

I also checked the docker driver implementation for RecoverTask and it doesn't decode the driver config as well.

2. For the network issue network hook fails after client restart w/ non-Docker driver hashicorp/nomad#9750, the fix: PR # 9757 would be going in the hashicorp/nomad codebase, so there is no change needed in the containerd-driver.