Improve network mode detection

[orlandohohmeier]

Adjust the network mode detection to properly handle cases without the network-scope label as the metrics collection would otherwise fail in those cases.

[orlandohohmeier]

@jkoelker @wavesoft would love a review from you on this one. Please note that it doesn't handle cases where the resources are reported on the executor as this would require additional requests which could degrade performance. Marathon pods launched on the host network would match this case but come with the network scope label and are thus already handled.

P.S. You will want to implement a similar detection in Edge-LB for the auto-pool feature to properly handle different network modes.

/cc @vinodkone

[jkoelker]

Logic wise LGTM, I'll have to look into the test failures later, it looks like the mocks just need to be updated.