DriverContext: Add the TaskGroup and the Job name

[jvrplmlmn]

Adding this fields to the DriverContext object, will allow us to pass
them to the drivers.

An use case for this, will be to emit tagged metrics in the drivers,
which contain all relevant information:

• Job
• TaskGroup
• Task
• ...

For more context, see the following discussion: #4185 (comment)

[dadgar]

@jvrplmlmn Looking good, check out the test failures:

https://travis-ci.org/hashicorp/nomad/jobs/369873910#L952
https://travis-ci.org/hashicorp/nomad/jobs/369873912#L551

[jvrplmlmn]

Thanks for the review @dadgar, seems like I missed to push 92cbfd0

Travis keeps failing the build in TestAllocRunner_RetryArtifact, but if I run that test locally it passes:

GOROOT=/usr/local/Cellar/go/1.9.2/libexec #gosetup
GOPATH=/Users/javier/go #gosetup
/usr/local/Cellar/go/1.9.2/libexec/bin/go test -c -i -o /private/var/folders/zw/lqcz80hj51b5q8_y56kc_1jr0000gn/T/___TestAllocRunner_RetryArtifact_in_alloc_runner_test_go github.com/hashicorp/nomad/client #gosetup
/private/var/folders/zw/lqcz80hj51b5q8_y56kc_1jr0000gn/T/___TestAllocRunner_RetryArtifact_in_alloc_runner_test_go -test.v -test.run ^TestAllocRunner_RetryArtifact$ #gosetup
	testlog.go:30: 23:16:28.552258 [DEBUG] client: starting task runners for alloc '15fd8da5-87cb-3bdc-8196-2766ec5806db'
	testlog.go:30: 23:16:28.552539 [DEBUG] client: starting task context for 'web' (alloc '15fd8da5-87cb-3bdc-8196-2766ec5806db')
	testlog.go:30: 23:16:28.552584 [DEBUG] client: starting task context for 'bad' (alloc '15fd8da5-87cb-3bdc-8196-2766ec5806db')
	testlog.go:30: 23:16:28.566418 [DEBUG] driver.mock: starting task "web"
	testlog.go:30: 23:16:28.566435 [TRACE] client: alloc 15fd8da5-87cb-3bdc-8196-2766ec5806db task web could not detect a driver IP
	testlog.go:30: 23:16:28.566534 [TEST] mock_consul: RegisterTask("15fd8da5-87cb-3bdc-8196-2766ec5806db", "web", *driver.mockDriverHandle, 68934a3e9455fa72420237eb05902327)
	testlog.go:30: 23:16:28.578283 [DEBUG] client: failed to download artifact "http://127.0.0.1:0/foo/bar/baz": Get http://127.0.0.1:0/foo/bar/baz: dial tcp 127.0.0.1:0: connect: can't assign requested address
	testlog.go:30: 23:16:28.578348 [INFO] client: Restarting task "bad" for alloc "15fd8da5-87cb-3bdc-8196-2766ec5806db" in 4.690040753s
	testlog.go:30: 23:16:28.578478 [TEST] mock_consul: RemoveTask("15fd8da5-87cb-3bdc-8196-2766ec5806db", "bad")
	testlog.go:30: 23:16:29.567708 [DEBUG] driver.mock: finished running task "web"
	testlog.go:30: 23:16:29.567911 [INFO] client: task "web" for alloc "15fd8da5-87cb-3bdc-8196-2766ec5806db" completed successfully
	testlog.go:30: 23:16:29.567941 [INFO] client: Not restarting task: web for alloc: 15fd8da5-87cb-3bdc-8196-2766ec5806db 
	testlog.go:30: 23:16:29.568134 [TEST] mock_consul: RemoveTask("15fd8da5-87cb-3bdc-8196-2766ec5806db", "web")
	testlog.go:30: 23:16:33.271376 [DEBUG] client: failed to download artifact "http://127.0.0.1:0/foo/bar/baz": Get http://127.0.0.1:0/foo/bar/baz: dial tcp 127.0.0.1:0: connect: can't assign requested address
	testlog.go:30: 23:16:33.271440 [INFO] client: Not restarting task: bad for alloc: 15fd8da5-87cb-3bdc-8196-2766ec5806db 
	testlog.go:30: 23:16:33.271488 [DEBUG] client: task "bad" failed, destroying other tasks in task group: [web]
	testlog.go:30: 23:16:33.271601 [TEST] mock_consul: RemoveTask("15fd8da5-87cb-3bdc-8196-2766ec5806db", "bad")
	testlog.go:30: 23:16:33.271670 [DEBUG] client: task "bad" failed, destroying other tasks in task group: [web]

Process finished with exit code 0

[github-actions]

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