Fix issues in containerd-driver

[shishir-a412ed]

We found 3 issues while testing failure scenarios in containerd i.e. If containerd goes down (or restarts), how does nomad-driver-containerd handles that situation:

Issues:

1. When containerd-driver makes a gRPC call to containerd e.g. during fingerprinting operation, a context timeout must be set. Right now, without the timeout, if containerd goes down, that call never returns leaving containerd-driver in a hung state.

2. handleWait() throwing a nil pointer exception: Sending an empty return to nomad client results in nomad client dereferencing a nil pointer which results in a nil pointer exception.

3. Issue in recovering task if nomad/nomad-driver-containerd restarts. This might also be related to the networking error we are observing

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
Dec 17 18:47:14 ip-10-102-98-114 nomad[27030]:  client.alloc_runner.task_runner: error destroying unrecoverable task: alloc_id=2d6b365f-cc89-b230-2af9-a37ccbf0f6c8 task=adaas-task error="rpc error: code = Unknown desc = task not found for given id" task_id=2d6b365f-cc89-b230-2af9-a37ccbf0f6c8/adaas-task/43e03bbd
2020-12-16T17:20:11-08:00  Setup Failure  failed to setup alloc: pre-run hook "network" failed: failed to create network for alloc: open /var/run/netns/d30ebc57-6ed4-9dc0-c604-e2fea98257f1: operation not permitted

This PR addresses (1) and (2). I will open a separate PR for Issue (3).

[shishir-a412ed]

@shivdudhani Can you please review?

[shivdudhani]

LGTM!