if docker can't connect during InitializeFSContext , do not add image dev info to real fs info label cache

[Dingshujie]

when cadvisor Initialize FS Context, docker restart or something wrong happen, context.Dokcer.Driver set to "", now default use /var/lib/docker device usage as image fs usage. if we use device mapper as storage driver, we may get wrong image fs usage inforamation， so may be lead kubelet image gc manager to get a wrong information。

[k8s-ci-robot]

Hi @Dingshujie. Thanks for your PR.

I'm waiting for a google member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[Dingshujie]

@bobbypage can you review this PR?

[bobbypage]

/ok-to-test

[bobbypage]

Looks like tests are failing?

I0908 21:49:27.339] --- FAIL: TestAddDockerImagesLabel (0.00s)
I0908 21:49:27.339]     fs_test.go:509: single partition, no dedicated image fs: docker device: expected "/dev/root", got ""
I0908 21:49:27.340]     fs_test.go:509: multiple mounts - innermost check: docker device: expected "/dev/sdb2", got ""
I0908 21:49:27.340]     fs_test.go:509: root fs inside container, docker-images bindmount: docker device: expected "/dev/sda1", got ""
I0908 21:49:27.340]     fs_test.go:509: [overlay2] root fs inside container - /var/lib/docker bindmount: docker device: expected "/dev/sdb2", got ""
I0908 21:49:27.341] FAIL

https://prow.k8s.io/view/gs/kubernetes-jenkins/pr-logs/pull/google_cadvisor/2934/pull-cadvisor-e2e/1435721017686233088/

[Dingshujie]

/retest

[Dingshujie]

Looks like tests are failing?

I0908 21:49:27.339] --- FAIL: TestAddDockerImagesLabel (0.00s)
I0908 21:49:27.339]     fs_test.go:509: single partition, no dedicated image fs: docker device: expected "/dev/root", got ""
I0908 21:49:27.340]     fs_test.go:509: multiple mounts - innermost check: docker device: expected "/dev/sdb2", got ""
I0908 21:49:27.340]     fs_test.go:509: root fs inside container, docker-images bindmount: docker device: expected "/dev/sda1", got ""
I0908 21:49:27.340]     fs_test.go:509: [overlay2] root fs inside container - /var/lib/docker bindmount: docker device: expected "/dev/sdb2", got ""
I0908 21:49:27.341] FAIL

https://prow.k8s.io/view/gs/kubernetes-jenkins/pr-logs/pull/google_cadvisor/2934/pull-cadvisor-e2e/1435721017686233088/

test case not set driver name, so lead test failed.
now fixed

[bobbypage]

thanks, LGTM.