Enable Logging tests for Travis

[stephenplusplus]

RE: #865 (comment)

Currently, Travis is skipping these system tests because it can't auth as a user:

Logging
    sinks (user auth operations)
      - should create a sink with a Bucket destination
      - should create a sink with a Dataset destination
      - should create a sink with a Topic destination
      metadata
        - should set metadata
        - should set a filter
      listing sinks
        - should list sinks
        - should list sinks as a stream
        - should get metadata

@jgeewax, I think you mentioned there might be a way to auth these requests with a service account?

[stephenplusplus]

@jgeewax wdyt?

[jgeewax]

I don't have an answer just yet. @rimey - do you happen to know whether we can do logging admin-type operations as a service account?

[rimey]

I don't see why not. gcloud-python does it.

[stephenplusplus]

Cool, looks like things have changed: #1282.